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第 8 章 The XFS File System

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 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 第 6 章 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)
This allows the system to associate several additional name/value pairs per file.
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.

8.1. Creating an XFS File System
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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.

例 8.1. 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
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注意

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 第 8.4 节 “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.
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
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For more information about creating XFS file systems, refer to man mkfs.xfs.
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