RHEL 6 Storage Admin - Chapter 9. The XFS File System

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Chapter 9. The XFS File System

9.1. Creating an XFS File System
9.2. Mounting an XFS File System
9.3. XFS Quota Management
9.4. Increasing the Size of an XFS File System
9.5. Repairing an XFS File System
9.6. Suspending an XFS File System
9.7. Backup and Restoration of XFS File Systems
9.8. Other XFS File System Utilities

XFS is a highly scalable, high-performance file system which was originally designed at Silicon Graphics, Inc. It was created to support extremely large filesystems (up to 16 exabytes), files (8 exabytes) and directory structures (tens of millions of entries).

Main Features

XFS supports metadata journaling, which facilitates quicker crash recovery. The XFS file system can also be defragmented and enlarged while mounted and active. In addition, Red Hat Enterprise Linux 6 supports backup and restore utilities specific to XFS.

Allocation Features

XFS features the following allocation schemes:

Extent-based allocation
Stripe-aware allocation policies
Delayed allocation
Space pre-allocation

Delayed allocation and other performance optimizations affect XFS the same way that they do ext4. Namely, a program's writes to a an XFS file system are not guaranteed to be on-disk unless the program issues an fsync() call afterwards.

For more information on the implications of delayed allocation on a file system, refer to Allocation Features in Chapter 7, The Ext4 File System. The workaround for ensuring writes to disk applies to XFS as well.

Other XFS Features

The XFS file system also supports the following:

Extended attributes (xattr), which allows the system to associate several additional name/value pairs per file.
Quota journalling, which avoids the need for lengthy quota consistency checks after a crash.
Project/directory quotas, allowing quota restrictions over a directory tree.
Subsecond timestamps

9.1. Creating an XFS File System

To create an XFS file system, use the mkfs.xfs /dev/device command. In general, the default options are optimal for common use.

When using mkfs.xfs on a block device containing an existing file system, use the -f option to force an overwrite of that file system.

Below is a sample output of the mkfs.xfs command:

meta-data=/dev/device            isize=256    agcount=4, agsize=3277258 blks
         =                       sectsz=512   attr=2
data     =                       bsize=4096   blocks=13109032, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0
log      =internal log           bsize=4096   blocks=6400, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0

Note

After an XFS file system is created, its size cannot be reduced. However, it can still be enlarged using the xfs_growfs command (refer to Section 9.4, “Increasing the Size of an XFS File System”).

For striped block devices (e.g., RAID5 arrays), the stripe geometry can be specified at the time of file system creation. Using proper stripe geometry greatly enhances the performance of an XFS filesystem.

When creating filesystems on lvm or md volumes, mkfs.xfs chooses an optimal geometry. This may also be true on some hardware RAIDs which export geometry information to the operating system.

To specify stripe geometry, use the following mkfs.xfs sub-options:

su=value: Specifies a stripe unit or RAID chunk size. The value must be specified in bytes, with an optional k, m, or g suffix.
sw=value: Specifies the number of data disks in a RAID device, or the number of stripe units in the stripe.

The following example specifies a chunk size of 64k on a RAID device containing 4 stripe units:

mkfs.xfs -d su=64k,sw=4 /dev/device

For more information about creating XFS file systems, refer to man mkfs.xfs.