| Storage Administration GuideChapter 25. Device Mapper Multipathing and Virtual StorageRed Hat Enterprise Linux 6 also supports DM-Multipath and virtual storage. Both features are documented in detail in the Red Hat books DM Multipath and Virtualization Administration Guide. Red Hat Enterprise Linux 6 supports the following file systems/online storage methods for virtual storage: Fibre Channel iSCSI NFS GFS2
Virtualization in Red Hat Enterprise Linux 6 uses libvirt to manage virtual instances. The libvirt utility uses the concept of storage pools to manage storage for virtualized guests. A storage pool is storage that can be divided up into smaller volumes or allocated directly to a guest. Volumes of a storage pool can be allocated to virtualized guests. There are two categories of storage pools available: - Local storage pools
Local storage covers storage devices, files or directories directly attached to a host. Local storage includes local directories, directly attached disks, and LVM Volume Groups. - Networked (shared) storage pools
Networked storage covers storage devices shared over a network using standard protocols. It includes shared storage devices using Fibre Channel, iSCSI, NFS, GFS2, and SCSI RDMA protocols, and is a requirement for migrating guest virtualized guests between hosts.
For comprehensive information on the deployment and configuration of virtual storage instances in your environment, please refer to the Virtualization Storage section of the Virtualization guide provided by Red Hat. Device Mapper Multipathing (DM-Multipath) is a feature that allows you to configure multiple I/O paths between server nodes and storage arrays into a single device. These I/O paths are physical SAN connections that can include separate cables, switches, and controllers. Multipathing aggregates the I/O paths, creating a new device that consists of the aggregated paths. DM-Multipath are used primarily for the following reasons: - Redundancy
DM-Multipath can provide failover in an active/passive configuration. In an active/passive configuration, only half the paths are used at any time for I/O. If any element of an I/O path (the cable, switch, or controller) fails, DM-Multipath switches to an alternate path. - Improved Performance
DM-Multipath can be configured in active/active mode, where I/O is spread over the paths in a round-robin fashion. In some configurations, DM-Multipath can detect loading on the I/O paths and dynamically re-balance the load.
For comprehensive information on the deployment and configuration of DM-Multipath in your environment, please refer to the Using DM-Multipath guide provided by Red Hat. | Revision History |
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| Revision 2-11 | Mon Feb 18 2013 | Jacquelynn East | | Version for 6.4 GA release. |
| | Revision 2-10 | Mon Feb 18 2013 | Jacquelynn East | | BZ#894891 Updated pNFS section. |
| | Revision 2-6 | Wed Jan 16 2013 | Jacquelynn East | | BZ#894891 Edited pNFS chapter to reflect 6.4 release notes. | | BZ#804784 Removed section on remove_on_dev_loss. |
| | Revision 2-5 | Tue Jan 15 2013 | Jacquelynn East | | BZ#894697 Updated sections regarding FCoE. |
| | Revision 2-4 | Mon Jan 14 2013 | Jacquelynn East | | BZ#894891 As pNFS is coming out of tech preview status, all references to this were removed. |
| | Revision 2-3 | Fri Oct 19 2012 | Jacquelynn East | | BZ#846498 Copy section from Performance Tuning Guide to File System Structure. |
| | Revision 2-1 | Fri Oct 19 2012 | Jacquelynn East | | Branched for 6.4 Beta. | | Created new edition based on significant structual reordering. |
| | Revision 1-56 | Fri Oct 12 2012 | Jacquelynn East | | Added information to the overview and respective parts. |
| | Revision 1-53 | Thu Oct 11 2012 | Jacquelynn East | | Rearranged and separated chapters into parts. |
| | Revision 1-52 | Tue Oct 2 2012 | Jacquelynn East | | BZ#784335 Copied chapters across from Online Storage Guide. |
| | Revision 1-50 | Tue Sep 25 2012 | Jacquelynn East | | BZ#804784 Removed warning regarding remove_on_dev_loss. |
| | Revision 1-49 | Tue SEP 18 2012 | Jacquelynn East | | Added information BZ#784405. |
| | Revision 1-48 | Thu Sep 13 2012 | Jacquelynn East | | Minor edit BZ#802859. | | Section on using NFS over UDP removed BZ#845601. |
| | Revision 1-47 | Wed Sep 5 2012 | Jacquelynn East | | | Revision 1-45 | Mon Jun 18 2012 | Jacquelynn East | |
This glossary defines common terms relating to file systems and storage used throughout the Storage Administration Guide. - Defragmentation
The act of reorganizing a file's data blocks so that they are more physically contiguous on disk. - Delayed Allocation
An allocator behavior in which disk locations are chosen when data is flushed to disk, rather than when the write occurs. This can generally lead to more efficient allocation because the allocator is called less often and with larger requests. - Extended Attributes
Name/Value metadata pairs which may be associated with a file. - Extent
A unit of file allocation, stored in the file's metadata as an offset, length pair. A single extent record can describe many contiguous blocks in a file. - File System Repair (fsck)
A method of verifying and repairing consistency of a file system's metadata. May be needed post-crash for non-journaling file systems, or after a hardware failure or kernel bug. - Fragmentation
The condition in which a file's data blocks are not allocated in contiguous physical (disk) locations for contiguous logical offsets within the file. File fragmentation can lead to poor performance in some situations, due to disk seek time. - Metadata Journaling
A method used to ensure that a file system's metadata is consistent even after a system crash. Metadata journaling can take different forms, but in each case a journal or log can be replayed after a crash, writing only consistent transactional changes to the disk. - Persistent Preallocation
A type of file allocation which chooses locations on disk, and marks these blocks as used regardless of when or if they are written. Until data is written into these blocks, reads will return 0s. Preallocation is performed with the fallocate() glibc function. - POSIX Access Control Lists (ACLs)
Metadata attached to a file which permits more fine-grained access controls. ACLS are often implemented as a special type of extended attribute. - Quota
A limit on block or inode usage of individual users and groups in a file system, set by the administrator. - Stripe Unit
Also sometimes referred to as stride or chunk-size. The stripe unit is the amount of data written to one component of striped storage before moving on to the next. Specified in byte or file system block units. - Stripe Width
The number of individual data stripe units in striped storage (excluding parity). Depending on the administrative tool used, may be specified in byte or file system block units, or in multiples of the stripe unit. - Stripe-aware allocation
An allocator behavior in which allocations and I/O are well-aligned to underlying striped storage. This depends on stripe information being available at mkfs time as well. Doing well-aligned allocation I/O can avoid inefficient read-modify-write cycles on the underlying storage. - Write Barriers
A method to enforce consistent I/O ordering on storage devices which have volatile write caches. Barriers must be used to ensure that after a power loss, the ordering guarantees required by metadata journaling are not violated due to the storage hardware writing out blocks from its volatile write cache in a different order than the operating system requested. See Also Metadata Journaling.
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