Chapter 4. The DM-Multipath Configuration File

By default, DM-Multipath provides configuration values for the most common uses of multipathing. In addition, DM-Multipath includes support for the most common storage arrays that support DM-Multipath. The default configuration values and the supported devices can be found in the /usr/share/doc/device-mapper-multipath-0.4.9/multipath.conf.defaults file.

You can override the default configuration values for DM-Multipath by editing the /etc/multipath.conf configuration file. If necessary, you can also add a storage array that is not supported by default to the configuration file.

Note

You can run set up multipathing in the initramfs file system. If you run multipath from the initramfs file system and you make any changes to the multipath configuration files, you must rebuild the initramfs file system for the changes to take effect. For information on rebuilding the initramfs file system with multipath, refer to Section 3.4, "Setting Up Multipathing in the initramfs File System".

This chapter provides information on parsing and modifying the multipath.conf file. It contains sections on the following topics:

Configuration file overview
Configuration file blacklist
Configuration file defaults
Configuration file multipaths
Configuration file devices

In the multipath configuration file, you need to specify only the sections that you need for your configuration, or that you wish to change from the default values specified in the multipath.conf.defaults file. If there are sections of the file that are not relevant to your environment or for which you do not need to override the default values, you can leave them commented out, as they are in the initial file.

The configuration file allows regular expression description syntax.

An annotated version of the configuration file can be found in /usr/share/doc/device-mapper-multipath-0.4.9/multipath.conf.annotated.

4.1. Configuration File Overview

The multipath configuration file is divided into the following sections:

blacklist: Listing of specific devices that will not be considered for multipath.
blacklist_exceptions: Listing of multipath candidates that would otherwise be blacklisted according to the parameters of the blacklist section.
defaults: General default settings for DM-Multipath.
multipaths: Settings for the characteristics of individual multipath devices. These values overwrite what is specified in the defaults and devices sections of the configuration file.
devices: Settings for the individual storage controllers. These values overwrite what is specified in the defaults section of the configuration file. If you are using a storage array that is not supported by default, you may need to create a devices subsection for your array.

When the system determines the attributes of a multipath device, first it checks the multipath settings, then the per devices settings, then the multipath system defaults.

4.2. Configuration File Blacklist

The blacklist section of the multipath configuration file specifies the devices that will not be used when the system configures multipath devices. Devices that are blacklisted will not be grouped into a multipath device.

In older releases of Red Hat Enterprise Linux, multipath always tried to create a multipath device for every path that was not explicitly blacklisted. In Red Hat Enterprise Linux 6, however, if the find_multipaths configuration parameter is set to yes, then multipath will create a device only if one of three conditions are met:

There are at least two non-blacklisted paths with the same WWID.
The user manually forces the creation of the device by specifying a device with the multipath command.
A path has the same WWID as a multipath device that was previously created (even if that multipath device does not currently exist). Whenever a multipath device is created, multipath remembers the WWID of the device so that it will automatically create the device again as soon as it sees a path with that WWID. This allows you to have multipath automatically choose the correct paths to make into multipath devices, without have to edit the multipath blacklist.
If you have previously created a multipath device without using the find_multipaths parameter and then you later set the parameter to yes, you may need to remove the WWIDs of any device you do not want created as a multipath device from the /etc/multipath/wwids file. The following shows a sample /etc/multipath/wwids file. The WWIDs are enclosed by slashes (/):
```
# Multipath wwids, Version : 1.0# NOTE: This file is automatically maintained by multipath and multipathd.# You should not need to edit this file in normal circumstances.## Valid WWIDs:/3600d0230000000000e13955cc3757802//3600d0230000000000e13955cc3757801//3600d0230000000000e13955cc3757800//3600d02300069c9ce09d41c31f29d4c00//SWINSYS  SF2372 0E13955CC3757802//3600d0230000000000e13955cc3757803/
```

With the find_multipaths parameter set to yes, you need to blacklist only the devices with multiple paths that you do not want to be multipathed. Because of this, it will generally not be necessary to blacklist devices.

If you do need to blacklist devices, you can do so according to the following criteria:

By WWID, as described in Section 4.2.1, "Blacklisting by WWID"
By device name, as described in Section 4.2.2, "Blacklisting By Device Name"
By device type, as described in Section 4.2.3, "Blacklisting By Device Type"

By default, a variety of device types are blacklisted, even after you comment out the initial blacklist section of the configuration file. For information, see Section 4.2.2, "Blacklisting By Device Name".

4.2.1. Blacklisting by WWID

You can specify individual devices to blacklist by their World-Wide IDentification with a wwid entry in the blacklist section of the configuration file.

The following example shows the lines in the configuration file that would blacklist a device with a WWID of 26353900f02796769.

blacklist {   wwid 26353900f02796769}

4.2.2. Blacklisting By Device Name

You can blacklist device types by device name so that they will not be grouped into a multipath device by specifying a devnode entry in the blacklist section of the configuration file.

The following example shows the lines in the configuration file that would blacklist all SCSI devices, since it blacklists all sd* devices.

blacklist {   devnode "^sd[a-z]"}

You can use a devnode entry in the blacklist section of the configuration file to specify individual devices to blacklist rather than all devices of a specific type. This is not recommended, however, since unless it is statically mapped by udev rules, there is no guarantee that a specific device will have the same name on reboot. For example, a device name could change from /dev/sda to /dev/sdb on reboot.

By default, the following devnode entries are compiled in the default blacklist; the devices that these entries blacklist do not generally support DM-Multipath. To enable multipathing on any of these devices, you would need to specify them in the blacklist_exceptions section of the configuration file, as described in Section 4.2.4, "Blacklist Exceptions".

blacklist {   devnode "^(ram|raw|loop|fd|md|dm-|sr|scd|st)[0-9]*"   devnode "^hd[a-z]"}

4.2.3. Blacklisting By Device Type

You can specify specific device types in the blacklist section of the configuration file with a device section. The following example blacklists all IBM DS4200 and HP devices.

blacklist {   device {   vendor  "IBM"   product "3S42"   #DS4200 Product 10   }   device {   vendor  "HP"   product "*"   }}

4.2.4. Blacklist Exceptions

You can use the blacklist_exceptions section of the configuration file to enable multipathing on devices that have been blacklisted by default.

For example, if you have a large number of devices and want to multipath only one of them (with the WWID of 3600d0230000000000e13955cc3757803), instead of individually blacklisting each of the devices except the one you want, you could instead blacklist all of them, and then allow only the one you want by adding the following lines to the /etc/multipath.conf file.

blacklist { wwid "*"}blacklist_exceptions { wwid "3600d0230000000000e13955cc3757803"}

When specifying devices in the blacklist_exceptions section of the configuration file, you must specify the exceptions in the same way they were specified in the blacklist. For example, a WWID exception will not apply to devices specified by a devnode blacklist entry, even if the blacklisted device is associated with that WWID. Similarly, devnode exceptions apply only to devnode entries, and device exceptions apply only to device entries.

4.3. Configuration File Defaults

The /etc/multipath.conf configuration file includes a defaults section that sets the user_friendly_names parameter to yes, as follows.

defaults { user_friendly_names yes}

This overwrites the default value of the user_friendly_names parameter.

The configuration file includes a template of configuration defaults. This section is commented out, as follows.

#defaults {#   udev_dir /dev#   polling_interval 5#   path_selector   "round-robin 0"#   path_grouping_policy failover#   getuid_callout  "/lib/dev/scsi_id --whitelisted --device=/dev/%n"#prioconst#path_checkerdirectio#rr_min_io1000#rr_weightuniform#failbackmanual#no_path_retryfail#user_friendly_namesno#}

To overwrite the default value for any of the configuration parameters, you can copy the relevant line from this template into the defaults section and uncomment it. For example, to overwrite the path_grouping_policy parameter so that it is multibus rather than the default value of failover, copy the appropriate line from the template to the initial defaults section of the configuration file, and uncomment it, as follows.

defaults { user_friendly_names yes path_grouping_policy multibus}

Table 4.1, "Multipath Configuration Defaults" describes the attributes that are set in the defaults section of the multipath.conf configuration file. These values are used by DM-Multipath unless they are overwritten by the attributes specified in the devices and multipaths sections of the multipath.conf file.

Note

As of the Red Hat Enterprise Linux 6.0 release, the mode, uid, and gid parameters have been deprecated. Permissions for device-mapper devices (including mulitpath mappings) are set by means of udev rules. There is a template file in /usr/share/doc/device-mapper-version called 12-dm-permissions.rules which you can use and place in the /etc/udev/rules.d directory for it to take effect.

Table 4.1. Multipath Configuration Defaults

Attribute Description

polling_interval Specifies the interval between two path checks in seconds. For properly functioning paths, the interval between checks will gradually increase to (4 * polling_interval). The default value is 5.

udev_dir The directory where udev device nodes are created. The default value is /dev.

multipath_dir The directory where the dynamic shared objects are stored. The default value is system dependent, commonly /lib/multipath.

find_multipaths

Defines the mode for setting up multipath devices. If this parameter is set to yes, then multipath will not try to create a device for every non-blacklisted path. Instead multipath will create a device only if one of three conditions are met:

- There are at least two non-blacklisted paths with the same WWID.

- The user manually forces the creation of the device by specifying a device with the multipath command.

- A path has the same WWID as a multipath device that was previously created. Whenever a multipath device is created with find_multipaths set, multipath remembers the WWID of the device so that it will automatically create the device again as soon as it sees a path with that WWID. This allows you to have multipath automatically choose the correct paths to make into multipath devices, without having to edit the multipath blacklist. For instructions on the procedure to follow if you have previously created multipath devices when the find_multipaths parameter was not set, see Section 4.2, "Configuration File Blacklist".

The default value is no.

verbosity The default verbosity. Higher values increase the verbosity level. Valid levels are between 0 and 6. The default value is 2.

path_selector

Specifies the default algorithm to use in determining what path to use for the next I/O operation. Possible values include:

round-robin 0: Loop through every path in the path group, sending the same amount of I/O to each.

queue-length 0: Send the next bunch of I/O down the path with the least number of outstanding I/O requests.

service-time 0: Send the next bunch of I/O down the path with the shortest estimated service time, which is determined by dividing the total size of the outstanding I/O to each path by its relative throughput.

The default value is round-robin 0.

path_grouping_policy

Specifies the default path grouping policy to apply to unspecified multipaths. Possible values include:

failover: 1 path per priority group.

multibus: all valid paths in 1 priority group.

group_by_serial: 1 priority group per detected serial number.

group_by_prio: 1 priority group per path priority value. Priorities are determined by callout programs specified as global, per-controller, or per-multipath options.

group_by_node_name: 1 priority group per target node name. Target node names are fetched in /sys/class/fc_transport/target*/node_name.

The default value is failover.

getuid_callout

Specifies the default program and arguments to call out to obtain a unique path identifier. An absolute path is required.

The default value is /lib/udev/scsi_id --whitelisted --device=/dev/%n.

prio

Specifies the default function to call to obtain a path priority value. For example, the ALUA bits in SPC-3 provide an exploitable prio value. Possible values include:

const: Set a priority of 1 to all paths.

emc: Generate the path priority for EMC arrays.

alua: Generate the path priority based on the SCSI-3 ALUA settings.

tpg_pref: Generate the path priority based on the SCSI-3 ALUA settings, using the preferred port bit.

ontap: Generate the path priority for NetApp arrays.

rdac: Generate the path priority for LSI/Engenio RDAC controller.

hp_sw: Generate the path priority for Compaq/HP controller in active/standby mode.

hds: Generate the path priority for Hitachi HDS Modular storage arrays.

The default value is const.

features The default extra features of multipath devices. The only existing feature is queue_if_no_path, which is the same as setting no_path_retry to queue. For information on issues that may arise when using this feature, see Section 5.6, "Issues with queue_if_no_path feature".

path_checker

Specifies the default method used to determine the state of the paths. Possible values include:

readsector0: Read the first sector of the device.

tur: Issue a TEST UNIT READY to the device.

emc_clariion: Query the EMC Clariion specific EVPD page 0xC0 to determine the path.

hp_sw: Check the path state for HP storage arrays with Active/Standby firmware.

rdac: Check the path stat for LSI/Engenio RDAC storage controller.

directio: Read the first sector with direct I/O.

The default value is directio.

failback

Manages path group failback.

A value of immediate specifies immediate failback to the highest priority path group that contains active paths.

A value of manual specifies that there should not be immediate failback but that failback can happen only with operator intervention.

A value of followover specifies that automatic failback should be performed when the first path of a path group becomes active. This keeps a node from automatically failing back when another node requested the failover.

A numeric value greater than zero specifies deferred failback, expressed in seconds.

The default value is manual.

rr_min_io Specifies the number of I/O requests to route to a path before switching to the next path in the current path group. This setting is only for systems running kernels older than 2.6.31. Newer systems should use rr_min_io_rq. The default value is 1000.

rr_min_io_rq Specifies the number of I/O requests to route to a path before switching to the next path in the current path group, using request-based device-mapper-multipath. This setting should be used on systems running current kernels. On systems running kernels older than 2.6.31, use rr_min_io. The default value is 1.

rr_weight If set to priorities, then instead of sending rr_min_io requests to a path before calling path_selector to choose the next path, the number of requests to send is determined by rr_min_io times the path's priority, as determined by the prio function. If set to uniform, all path weights are equal. The default value is uniform.

no_path_retry

A numeric value for this attribute specifies the number of times the system should attempt to use a failed path before disabling queueing.

A value of fail indicates immediate failure, without queueing.

A value of queue indicates that queueing should not stop until the path is fixed.

The default value is 0.

user_friendly_names If set to yes, specifies that the system should use the /etc/multipath/bindings file to assign a persistent and unique alias to the multipath, in the form of mpathn. If set to no, specifies that the system should use the WWID as the alias for the multipath. In either case, what is specified here will be overridden by any device-specific aliases you specify in the multipaths section of the configuration file. The default value is no.

queue_without_daemon If set to no, the multipathd daemon will disable queueing for all devices when it is shut down. The default value is no.

flush_on_last_del If set to yes, the multipathd daemon will disable queueing when the last path to a device has been deleted. The default value is no.

max_fds Sets the maximum number of open file descriptors that can be opened by multipath and the multipathd daemon. This is equivalent to the ulimit -n command. As of the Red Hat Enterprise Linux 6.3 release, the default value is max, which sets this to the system limit from /proc/sys/fs/nr_open. For earlier releases, if this is not set the maximum number of open file descriptors is taken from the calling process; it is usually 1024. To be safe, this should be set to the maximum number of paths plus 32, if that number is greater than 1024.

checker_timeout The timeout to use for path checkers that issue SCSI commands with an explicit timeout, in seconds. The default value is taken from sys/block/sdx/device/timeout.

fast_io_fail_tmo The number of seconds the SCSI layer will wait after a problem has been detected on an FC remote port before failing I/O to devices on that remote port. This value should be smaller than the value of dev_loss_tmo. Setting this to off will disable the timeout. The default value is determined by the OS.

dev_loss_tmo The number of seconds the SCSI layer will wait after a problem has been detected on an FC remote port before removing it from the system. Setting this to infinity will set this to 2147483647 seconds, or 68 years. The default value is determined by the OS.

hwtable_regex_match

(Red Hat Enterprise Linux Release 6.3 and later) Controls how multipath integrates the device configurations from the devices section of the configuration file with the built-in device configurations.

Each device configuration in the devices section of the multipath.conf file will either create its own device configuration or it will modify one of the built-in device configurations. Prior to Red Hat Enterprise Linux 6.3, if the vendor, product, and revision strings in a user's device configuration exactly matched those strings in a built-in device configuration, the built-in configuration was modified by the options in the user's configuration. Otherwise. the user's device configuration was treated as a new configuration.

If hwtable_regex_match is set to yes, a regular expression match is used instead. The vendor, product, and revision strings are all regular expressions. The user device configuration values for these options are matched against the built-in device configuration values. This match works the same way that an actual device's vendor product and revision strings are matched against a device configuration's strings to see which configuration should be used for the device. If the user's device configuration matches, then the built-in configuration is modified by the options in the user's configuration. Otherwise the user's device configuration is treated as a new configuration.

retain_attached_hw_handler (Red Hat Enterprise Linux Release 6.4 and later) If this parameter is set to yes and the scsi layer has already attached a hardware handler to the path device, multipath will not force the device to use the hardware_handler specified by the mutipath.conf file. If the scsi layer has not attached a hardware handler, multipath will continue to use its configured hardware handler as usual. The default value is no.

detect_prio (Red Hat Enterprise Linux Release 6.4 and later) If this is set to yes, multipath will first check if the device supports ALUA, and if so it will automatically assign the device the alua prioritizer. If the device doesn't support ALUA, it will determine the prioritizer as it always does. The default value is no.

4.4. Multipaths Device Configuration Attributes

Table 4.2, "Multipath Attributes" shows the attributes that you can set in the multipaths section of the multipath.conf configuration file for each specific multipath device. These attributes apply only to the one specified multipath. These defaults are used by DM-Multipath and override attributes set in the defaults and devices sections of the multipath.conf file.

Table 4.2. Multipath Attributes

Attribute Description

wwid Specifies the WWID of the multipath device to which the multipath attributes apply. This parameter is mandatory for this section of the multipath.conf file.

alias Specifies the symbolic name for the multipath device to which the multipath attributes apply. If you are using user_friendly_names, do not set this value to mpathn; this may conflict with an automatically assigned user friendly name and give you incorrect device node names.

path_grouping_policy

Specifies the default path grouping policy to apply to unspecified multipaths. Possible values include:

failover = 1 path per priority group

multibus = all valid paths in 1 priority group

group_by_serial = 1 priority group per detected serial number

group_by_prio = 1 priority group per path priority value

group_by_node_name = 1 priority group per target node name

path_selector

Specifies the default algorithm to use in determining what path to use for the next I/O operation. Possible values include:

round-robin 0: Loop through every path in the path group, sending the same amount of I/O to each.

queue-length 0: Send the next bunch of I/O down the path with the least number of outstanding I/O requests.

failback

Manages path group failback.

A value of immediate specifies immediate failback to the highest priority path group that contains active paths.

A value of manual specifies that there should not be immediate failback but that failback can happen only with operator intervention.

A numeric value greater than zero specifies deferred failback, expressed in seconds.

prio

Specifies the default function to call to obtain a path priority value. For example, the ALUA bits in SPC-3 provide an exploitable prio value. Possible values include:

const: Set a priority of 1 to all paths.

emc: Generate the path priority for EMC arrays.

alua: Generate the path priority based on the SCSI-3 ALUA settings.

tpg_pref: Generate the path priority based on the SCSI-3 ALUA settings, using the preferred port bit.

ontap: Generate the path priority for NetApp arrays.

rdac: Generate the path priority for LSI/Engenio RDAC controller.

hp_sw: Generate the path priority for Compaq/HP controller in active/standby mode.

hds: Generate the path priority for Hitachi HDS Modular storage arrays.

no_path_retry

A numeric value for this attribute specifies the number of times the system should attempt to use a failed path before disabling queueing.

A value of fail indicates immediate failure, without queueing.

A value of queue indicates that queueing should not stop until the path is fixed.

rr_min_io Specifies the number of I/O requests to route to a path before switching to the next path in the current path group. This setting is only for systems running kernels older that 2.6.31. Newer systems should use rr_min_io_rq. The default value is 1000.

flush_on_last_del If set to yes, then multipath will disable queueing when the last path to a device has been deleted.

user_friendly_names If set to yes, specifies that the system should use the /etc/multipath/bindings file to assign a persistent and unique alias to the multipath, in the form of mpathn. If set to no, specifies that the system should use use the WWID as the alias for the multipath. In either case, what is specified here will be overridden by any device-specific aliases you specify in the multipaths section of the configuration file.

The following example shows multipath attributes specified in the configuration file for two specific multipath devices. The first device has a WWID of 3600508b4000156d70001200000b0000 and a symbolic name of yellow.

The second multipath device in the example has a WWID of 1DEC_____321816758474 and a symbolic name of red. In this example, the rr_weight attributes is set to priorities.

multipaths {   multipath {  wwid  3600508b4000156d70001200000b0000  alias yellow  path_grouping_policy  multibus  path_selector "round-robin 0"  failback  manual  rr_weight priorities  no_path_retry 5   }   multipath {  wwid  1DEC_____321816758474  alias red  rr_weight priorities }}

4.5. Configuration File Devices

Table 4.3, "Device Attributes" shows the attributes that you can set for each individual storage device in the devices section of the multipath.conf configuration file. These attributes are used by DM-Multipath unless they are overwritten by the attributes specified in the multipaths section of the multipath.conf file for paths that contain the device. These attributes override the attributes set in the defaults section of the multipath.conf file.

Many devices that support multipathing are included by default in a multipath configuration. The values for the devices that are supported by default are listed in the multipath.conf.defaults file. You probably will not need to modify the values for these devices, but if you do you can overwrite the default values by including an entry in the configuration file for the device that overwrites those values. You can copy the device configuration defaults from the multipath.conf.defaults file for the device and override the values that you want to change.

To add a device to this section of the configuration file that is not configured automatically by default, you need to set the vendor and product parameters. You can find these values by looking at /sys/block/device_name/device/vendor and /sys/block/device_name/device/model where device_name is the device to be multipathed, as in the following example:

# cat /sys/block/sda/device/vendorWINSYS  # cat /sys/block/sda/device/modelSF2372

The additional parameters to specify depend on your specific device. If the device is active/active, you will usually not need to set additional parameters. You may want to set path_grouping_policy to multibus. Other parameters you may need to set are no_path_retry and rr_min_io, as described in Table 4.3, "Device Attributes".

If the device is active/passive, but it automatically switches paths with I/O to the passive path, you need to change the checker function to one that does not send I/O to the path to test if it is working (otherwise, your device will keep failing over). This almost always means that you set the path_checker to tur; this works for all SCSI devices that support the Test Unit Ready command, which most do.

If the device needs a special command to switch paths, then configuring this device for multipath requires a hardware handler kernel module. The current available hardware handler is emc. If this is not sufficient for your device, you may not be able to configure the device for multipath.

Table 4.3. Device Attributes

Attribute Description

vendor Specifies the vendor name of the storage device to which the device attributes apply, for example COMPAQ.

product Specifies the product name of the storage device to which the device attributes apply, for example HSV110 (C)COMPAQ.

revision Specifies the product revision identifier of the storage device.

product_blacklist Specifies a regular expression used to blacklist devices by product.

hardware_handler

Specifies a module that will be used to perform hardware specific actions when switching path groups or handling I/O errors. Possible values include:

1 emc: hardware handler for EMC storage arrays.

1 alua: hardware handler for SCSI-3 ALUA arrays.

1 hp_sw: hardware handler for Compaq/HP controllers.

1 rdac: hardware handler for the LSI/Engenio RDAC controllers.

path_grouping_policy

Specifies the default path grouping policy to apply to unspecified multipaths. Possible values include:

failover = 1 path per priority group

multibus = all valid paths in 1 priority group

group_by_serial = 1 priority group per detected serial number

group_by_prio = 1 priority group per path priority value

group_by_node_name = 1 priority group per target node name

getuid_callout Specifies the default program and arguments to call out to obtain a unique path identifier. An absolute path is required.

path_selector

Specifies the default algorithm to use in determining what path to use for the next I/O operation. Possible values include:

round-robin 0: Loop through every path in the path group, sending the same amount of I/O to each.

queue-length 0: Send the next bunch of I/O down the path with the least number of outstanding I/O requests.

path_checker

Specifies the default method used to determine the state of the paths. Possible values include:

readsector0: Read the first sector of the device.

tur: Issue a TEST UNIT READY to the device.

emc_clariion: Query the EMC Clariion specific EVPD page 0xC0 to determine the path.

hp_sw: Check the path state for HP storage arrays with Active/Standby firmware.

rdac: Check the path stat for LSI/Engenio RDAC storage controller.

directio: Read the first sector with direct I/O.

features The extra features of multipath devices. The only existing feature is queue_if_no_path, which is the same as setting no_path_retry to queue. For information on issues that may arise when using this feature, see Section 5.6, "Issues with queue_if_no_path feature".

prio

Specifies the default function to call to obtain a path priority value. For example, the ALUA bits in SPC-3 provide an exploitable prio value. Possible values include:

const: Set a priority of 1 to all paths.

emc: Generate the path priority for EMC arrays.

alua: Generate the path priority based on the SCSI-3 ALUA settings.

tpg_pref: Generate the path priority based on the SCSI-3 ALUA settings, using the preferred port bit.

ontap: Generate the path priority for NetApp arrays.

rdac: Generate the path priority for LSI/Engenio RDAC controller.

hp_sw: Generate the path priority for Compaq/HP controller in active/standby mode.

hds: Generate the path priority for Hitachi HDS Modular storage arrays.

failback

Manages path group failback.

A value of immediate specifies immediate failback to the highest priority path group that contains active paths.

A value of manual specifies that there should not be immediate failback but that failback can happen only with operator intervention.

A numeric value greater than zero specifies deferred failback, expressed in seconds.

no_path_retry

A numeric value for this attribute specifies the number of times the system should attempt to use a failed path before disabling queueing.

A value of fail indicates immediate failure, without queueing.

A value of queue indicates that queueing should not stop until the path is fixed.

rr_min_io Specifies the number of I/O requests to route to a path before switching to the next path in the current path group. This setting is only for systems running kernels older that 2.6.31. Newer systems should use rr_min_io_rq. The default value is 1000.

flush_on_last_del If set to yes, the multipathd daemon will disable queueing when the last path to a device has been deleted.

user_friendly_names If set to yes, specifies that the system should use the /etc/multipath/bindings file to assign a persistent and unique alias to the multipath, in the form of mpathn. If set to no, specifies that the system should use use the WWID as the alias for the multipath. In either case, what is specified here will be overridden by any device-specific aliases you specify in the multipaths section of the configuration file. The default value is no.

The following example shows a device entry in the multipath configuration file.

#}#device {#vendor"COMPAQ  "#product"MSA1000 "#path_grouping_policymultibus#path_checkertur#rr_weightpriorities#}#}

Chapter 5. DM-Multipath Administration and Troubleshooting

5.1. Resizing an Online Multipath Device
5.2. Moving root File Systems from a Single Path Device to a Multipath Device
5.3. Moving swap File Systems from a Single Path Device to a Multipath Device
5.4. The Multipath Daemon
5.5. Issues with Large Number of LUNs
5.6. Issues with queue_if_no_path feature
5.7. Multipath Command Output
5.8. Multipath Queries with multipath Command
5.9. Multipath Command Options
5.10. Determining Device Mapper Entries with the dmsetup Command
5.11. Troubleshooting with the multipathd Interactive Console

This chapter will provide information on administering DM-Multipath on a running system. It includes sections on the following topics:

Resizing an online multipath device
Moving the root device from a single-path device to a multipath device
Moving the swap device from a single path device to a multipath device
The multipath daemon
Issues with large number of LUNs
Issues with queue_if_no_path feature
multipath command output
Multipath queries with the multipath command
multipath command options
Multipath queries with the dmsetup command
Troubleshooting with the multipathd interactive console

5.1. Resizing an Online Multipath Device

If you need to resize an online multipath device, use the following procedure.

Resize your physical device.
Use the following command to find the paths to the LUN:
```
# multipath -l
```
Resize your paths. For SCSI devices, writing a 1 to the rescan file for the device causes the SCSI driver to rescan, as in the following command:
```
# echo 1 > /sys/block/device_name/device/rescan 
```
Resize your multipath device by running the multipathd resize command:
```
# multipathd -k'resize map mpatha'
```
Resize the file system (assuming no LVM or DOS partitions are used):
```
# resize2fs /dev/mapper/mpatha
```

5.2. Moving root File Systems from a Single Path Device to a Multipath Device

If you have installed your system on a single-path device and later add another path to the root file system, you will need to move your root file system to a multipathed device. This section documents the procedure for moving from a single-path to a multipathed device.

After ensuring that you have installed the device-mapper-multipath package, perform the following procedure:

Execute the following command to create the /etc/multipath.conf configuration file, load the multipath module, and set chkconfig for the multipathd to on:
```
# mpathconf --enable
```
For further information on using the mpathconf command to set up multipathing, see Section 3.1, "Setting Up DM-Multipath".
Edit the blacklist and blacklist_exceptions sections of the /etc/multipath.conf file, as described in Section 4.2, "Configuration File Blacklist".
To confirm that your configuration file is set up correctly, you can run the /sbin/multipath command with the -v3 option to check whether the multipath daemon tried to create a multipath device over your root device. The command will fail since the root the device is in use, but the output from the command should show the root device in the paths list.
You should look in the command output for a line of the following format:
```
WWID  H:B:T:L devname MAJOR:MINOR
```
For example, if your root file system is set up on sda or one of its partitions, you would see a line in the output such as the following:
```
===== paths list =====...1ATA WDC WD800JD-75MSA3   WD-WMAM9F 1:0:0:0 sda 8:0...
```
Later in the output, you should see the root device assigned to a multipath device:
```
time | devname: ownership set to mpathdev
```
For example, the output may appear as follows:
```
Jun 14 06:48:21 | sda: ownership set to mpatha
```
You will also see an indication that the command failed to create the multipath device with a line of the following format:
```
time | mpathdev: domap (0) failure for create/reload map
```
In the example noted above, you would see the following line in the command output:
```
Jun 14 06:48:21 | mpatha: domap (0) failure for create/reload map
```
To rebuild the initramfs file system with multipath, execute the dracut command with the following options:
```
# dracut --force --add multipath --include /etc/multipath /etc/multipath
```
If your root device is not an LVM volume and it is mounted by device name, you may need to edit the fstab file to switch to the appropriate multipath device name. If your root device is an LVM device or is mounted by UUID or something else, this step is not necessary.
1. Use the procedure described in Step 3 of running the /sbin/multipath command with the -v3 to determine the WWID of the root device.
2. Set up an alias for the root device in the /etc/multipath.conf file:
```
multipaths { multipath { wwid WWID_of_root_device alias rootdev }}
```
3. Edit the /etc/fstab and replace the old device path to the root device with the multipath device.
  For example, if you had the following entry in the /etc/fstab file:
```
/dev/sda1 /   ext4 defaults 1 1
```
  You would change the entry to the following:
```
/dev/mapper/rootdev / ext4 defaults 1 1
```
If you need to edit the /etc/fstab file, you will also need to edit the /etc/grub/grub.conf file and change the root parameter from root=/dev/sda1 to root=/dev/mapper/rootdev.
The following example shows what this grub.conf file entry would look like before you edit it.
```
title Red Hat Enterprise Linux FoundationServer (2.6.32-71.24.1.el6.x86_64) root (hd0,0) kernel /vmlinuz-2.6.32-71.24.1.el6.x86_64 ro root=/dev/sda1 rd_NO_LUKSrd_NO_MD rd_NO_DM LANG=en_US.UTF-8 SYSFONT=latarcyrheb-sun16 KEYTABLE=usconsole=ttyS0,115200n8 crashkernel=auto initrd /initramfs-2.6.32-71.24.1.el6.x86_64.img
```
The following example shows what the grub.conf file entry would look like after you edit it.
```
title Red Hat Enterprise Linux FoundationServer (2.6.32-71.24.1.el6.x86_64) root (hd0,0) kernel /vmlinuz-2.6.32-71.24.1.el6.x86_64 ro root=/dev/mapper/rootdevrd_NO_LUKS rd_NO_MD rd_NO_DM LANG=en_US.UTF-8 SYSFONT=latarcyrheb-sun16KEYTABLE=us console=ttyS0,115200n8 crashkernel=auto initrd /initramfs-2.6.32-71.24.1.el6.x86_64.img
```
Shut the machine down.
Configure the FC switch so that other paths are visible to the machine.
Boot the machine.
Check whether the root file system ('/') is on the multipathed device.

5.3. Moving swap File Systems from a Single Path Device to a Multipath Device

By default, swap devices are set up as logical volumes. This does not require any special configuration for configuring them as multipath devices as long as you set up multipathing on the physical volumes that constitute the logical volume group. If your swap device is not an LVM volume, however, and it is mounted by device name, you may need to edit the fstab file to switch to the appropriate multipath device name.

Determine the WWID number of the swap device by running the /sbin/multipath command with the -v3 option. The output from the command should show the swap device in the paths list.
You should look in the command output for a line of the following format, showing the swap device:
```
WWID  H:B:T:L devname MAJOR:MINOR
```
For example, if your swap file system is set up on sda or one of its partitions, you would see a line in the output such as the following:
```
===== paths list =====...1ATA WDC WD800JD-75MSA3   WD-WMAM9F 1:0:0:0 sda 8:0...
```

Set up an alias for the swap device in the /etc/multipath.conf file:

multipaths { multipath { wwid WWID_of_swap_device alias swapdev }}

Edit the /etc/fstab and replace the old device path to the root device with the multipath device.
For example, if you had the following entry in the /etc/fstab file:
```
/dev/sda2 swap ext4 defaults 0 0
```
You would change the entry to the following:
```
/dev/mapper/swapdev swap  ext4 defaults 0 0
```

5.4. The Multipath Daemon

If you find you have trouble implementing a multipath configuration, you should ensure that the multipath daemon is running, as described in Chapter 3, Setting Up DM-Multipath. The multipathd daemon must be running in order to use multipathed devices.

5.5. Issues with Large Number of LUNs

When a large number of LUNs are added to a node, using multipathed devices can significantly increase the time it takes for the udev device manager to create device nodes for them. If you experience this problem, you can correct it by deleting the following line in /etc/udev/rules.d/40-multipath.rules:

KERNEL!="dm-[0-9]*", ACTION=="add", PROGRAM=="/bin/bash -c '/sbin/lsmod | /bin/grep ^dm_multipath'", RUN+="/sbin/multipath -v0 %M:%m"

This line causes the udev device manager to run multipath every time a block device is added to the node. Even with this line removed, the multipathd daemon will still automatically create multipathed devices, and multipath will still be called during the boot process for nodes with multipathed root file systems. The only change is that multipathed devices will not be automatically created when the multipathd daemon is not running, which should not be a problem for the vast majority of multipath users.

5.6. Issues with queue_if_no_path feature

If features "1 queue_if_no_path" is specified in the /etc/multipath.conf file, then any process that issues I/O will hang until one or more paths are restored. To avoid this, set the no_path_retry N parameter in the /etc/multipath.conf file (where N is the number of times the system should retry a path).

When you set the no_path_retry parameter, remove the features "1 queue_if_no_path" option from the /etc/multipath.conf file as well. If, however, you are using a multipathed device for which the features "1 queue_if_no_path" option is set as a compiled-in default, as it is for many SAN devices, you must explicitly add features "0" to override this default. You can do this by copying the existing devices section for your device from /usr/share/doc/device-mapper-multipath-0.4.9/multipath.conf.defaults into /etc/multipath.conf and editing it to suit your needs.

If you need to use the features "1 queue_if_no_path" option and you experience the issue noted here, use the dmsetup command to edit the policy at runtime for a particular LUN (that is, for which all the paths are unavailable). For example, if you want to change the policy on the multipath device mpathc from "queue_if_no_path" to "fail_if_no_path", execute the following command.

dmsetup message mpathc 0 "fail_if_no_path"

Note that you must specify the mpathn alias rather than the path.

5.7. Multipath Command Output

When you create, modify, or list a multipath device, you get a printout of the current device setup. The format is as follows.

For each multipath device:

 action_if_any: alias (wwid_if_different_from_alias) dm_device_name_if_known vendor,product size=size features='features' hwhandler='hardware_handler' wp=write_permission_if_known

For each path group:

-+- policy='scheduling_policy' prio=prio_if_known status=path_group_status_if_known

For each path:

 `- host:channel:id:lun devnode major:minor dm_status_if_known path_status online_status

For example, the output of a multipath command might appear as follows:

3600d0230000000000e13955cc3757800 dm-1 WINSYS,SF2372size=269G features='0' hwhandler='0' wp=rw|-+- policy='round-robin 0' prio=1 status=active| `- 6:0:0:0 sdb 8:16  active ready  running`-+- policy='round-robin 0' prio=1 status=enabled  `- 7:0:0:0 sdf 8:80  active ready  running

If the path is up and ready for I/O, the status of the path is ready or ghost. If the path is down, the status is faulty or shaky. The path status is updated periodically by the multipathd daemon based on the polling interval defined in the /etc/multipath.conf file.

The dm status is similar to the path status, but from the kernel's point of view. The dm status has two states: failed, which is analogous to faulty, and active which covers all other path states. Occasionally, the path state and the dm state of a device will temporarily not agree.

The possible values for online_status are running and offline. A status of offline means that this SCSI device has been disabled.

Note

When a multipath device is being created or modified, the path group status, the dm device name, the write permissions, and the dm status are not known. Also, the features are not always correct.

5.8. Multipath Queries with multipath Command

You can use the -l and -ll options of the multipath command to display the current multipath configuration. The -l option displays multipath topology gathered from information in sysfs and the device mapper. The -ll option displays the information the -l displays in addition to all other available components of the system.

When displaying the multipath configuration, there are three verbosity levels you can specify with the -v option of the multipath command. Specifying -v0 yields no output. Specifying -v1 outputs the created or updated multipath names only, which you can then feed to other tools such as kpartx. Specifying -v2 prints all detected paths, multipaths, and device maps.

The following example shows the output of a multipath -l command.

# multipath -l3600d0230000000000e13955cc3757800 dm-1 WINSYS,SF2372size=269G features='0' hwhandler='0' wp=rw|-+- policy='round-robin 0' prio=1 status=active| `- 6:0:0:0 sdb 8:16  active ready  running`-+- policy='round-robin 0' prio=1 status=enabled  `- 7:0:0:0 sdf 8:80  active ready  running

The following example shows the output of a multipath -ll command.

# multipath -ll3600d0230000000000e13955cc3757801 dm-10 WINSYS,SF2372size=269G features='0' hwhandler='0' wp=rw|-+- policy='round-robin 0' prio=1 status=enabled| `- 19:0:0:1 sdc 8:32  active ready  running`-+- policy='round-robin 0' prio=1 status=enabled  `- 18:0:0:1 sdh 8:112 active ready  running3600d0230000000000e13955cc3757803 dm-2 WINSYS,SF2372size=125G features='0' hwhandler='0' wp=rw`-+- policy='round-robin 0' prio=1 status=active  |- 19:0:0:3 sde 8:64  active ready  running  `- 18:0:0:3 sdj 8:144 active ready  running

5.9. Multipath Command Options

Table 5.1, "Useful multipath Command Options" describes some options of the multipath command that you may find useful.

Table 5.1. Useful multipath Command Options

Option	Description
`-l`	Display the current multipath configuration gathered from `sysfs` and the device mapper.
`-ll`	Display the current multipath configuration gathered from `sysfs`, the device mapper, and all other available components on the system.
`-f device`	Remove the named multipath device.
`-F`	Remove all unused multipath devices.

5.10. Determining Device Mapper Entries with the dmsetup Command

You can use the dmsetup command to find out which device mapper entries match the multipathed devices.

The following command displays all the device mapper devices and their major and minor numbers. The minor numbers determine the name of the dm device. For example, a minor number of 3 corresponds to the multipathed device /dev/dm-3.

# dmsetup lsmpathd  (253:4)mpathep1 (253:12)mpathfp1 (253:11)mpathb  (253:3)mpathgp1 (253:14)mpathhp1 (253:13)mpatha  (253:2)mpathh  (253:9)mpathg  (253:8)VolGroup00-LogVol01 (253:1)mpathf  (253:7)VolGroup00-LogVol00 (253:0)mpathe  (253:6)mpathbp1 (253:10)mpathd  (253:5)

5.11. Troubleshooting with the multipathd Interactive Console

The multipathd -k command is an interactive interface to the multipathd daemon. Entering this command brings up an interactive multipath console. After entering this command, you can enter help to get a list of available commands, you can enter a interactive command, or you can enter CTRL-D to quit.

The multipathd interactive console can be used to troubleshoot problems you may be having with your system. For example, the following command sequence displays the multipath configuration, including the defaults, before exiting the console.

# multipathd -k> > show config> > CTRL-D

The following command sequence ensures that multipath has picked up any changes to the multipath.conf,

# multipathd -k> > reconfigure> > CTRL-D

Use the following command sequence to ensure that the path checker is working properly.

# multipathd -k> > show paths> > CTRL-D

DM Multipath

Chapter 4. The DM-Multipath Configuration File

4.1. Configuration File Overview

4.2. Configuration File Blacklist

4.2.1. Blacklisting by WWID

4.2.2. Blacklisting By Device Name

4.2.3. Blacklisting By Device Type

4.2.4. Blacklist Exceptions

4.3. Configuration File Defaults

4.4. Multipaths Device Configuration Attributes

4.5. Configuration File Devices

Chapter 5. DM-Multipath Administration and Troubleshooting

5.1. Resizing an Online Multipath Device

5.2. Moving root File Systems from a Single Path Device to a Multipath Device

5.3. Moving swap File Systems from a Single Path Device to a Multipath Device

5.4. The Multipath Daemon

5.5. Issues with Large Number of LUNs

5.6. Issues with queue_if_no_path feature

5.7. Multipath Command Output

5.8. Multipath Queries with multipath Command

5.9. Multipath Command Options

5.10. Determining Device Mapper Entries with the dmsetup Command

5.11. Troubleshooting with the multipathd Interactive Console

Revision History

Index

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