Chapter 3. The XFS File System

XFS is a highly scalable, high-performance file system which was originally designed at Silicon Graphics, Inc. XFS is the default file system for Red Hat Enterprise Linux 7.
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 7 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 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 (ext4 and XFS), refer to Allocation Features in Chapter 5, The Ext4 File System.

Note

Creating or expanding files occasionally fails with an unexpected ENOSPC write failure even though the disk space appears to be sufficient. This is due to XFS's performance-oriented design. In practice, it does not become a problem since it only occurs if remaining space is only a few blocks.
Other XFS Features
The XFS file system also supports the following:
Extended attributes (xattr)
This allows the system to associate several additional name/value pairs per file. It is enabled by default.
Quota journaling
This avoids the need for lengthy quota consistency checks after a crash.
Project/directory quotas
This allows quota restrictions over a directory tree.
Subsecond timestamps
This allows timestamps to go to the subsecond.
Default atime behavior is relatime
Relatime is on by default for XFS. It has almost no overhead compared to noatime while still maintaining sane atime values.

3.1. 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.

Example 3.1. mkfs.xfs command output

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 3.4, “Increasing the Size of an XFS File System”).
For striped block devices (for example, 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 that export geometry information to the operating system.
If the device exports stripe geometry information, mkfs (for ext3, ext4, and xfs) will automatically use this geometry. If stripe geometry is not detected by mkfs and even though the storage does, in fact, have stripe geometry, it is possible to manually specify it at mkfs time using the following 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 and the Red Hat Enterprise Linux Performance Tuning Guide, chapter Basic Tuning for XFS.