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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 of XFS
- XFS supports metadata journaling, which facilitates quicker crash recovery.
- The XFS file system can 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 anfsync()call afterwards.For more information on the implications of delayed allocation on a file system (ext4 and XFS), see 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.
- Extended attributes (
- Default
atimebehavior isrelatime Relatimeis on by default for XFS. It has almost no overhead compared tonoatimewhile still maintaining saneatimevalues.
3.1. Creating an XFS File System
- To create an XFS file system, use the following command:
#mkfs.xfs block_device- Replace block_device with the path to a block device. For example,
/dev/sdb1,/dev/disk/by-uuid/05e99ec8-def1-4a5e-8a9d-5945339ceb2a, or/dev/my-volgroup/my-lv. - In general, the default options are optimal for common use.
- When using
mkfs.xfson a block device containing an existing file system, add the-foption to overwrite that file system.
Example 3.1. mkfs.xfs Command Output
Following 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. For more information, see Section 3.4, “Increasing the Size of an XFS File System”).
Striped Block Devices
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, the
mkfs utility (for ext3, ext4, and xfs) will automatically use this geometry. If stripe geometry is not detected by the mkfs utility and even though the storage does, in fact, have stripe geometry, it is possible to manually specify it when creating the file system using the following options:
- su=value
- Specifies a stripe unit or RAID chunk size. The
valuemust be specified in bytes, with an optionalk,m, orgsuffix. - 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/block_device
Additional Resources
For more information about creating XFS file systems, see:
- The mkfs.xfs(8) man page
