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XFS FAQ
Quick links:
http://oss.sgi.com/projects/xfs/faq.html
Many thanks to earlier maintainers of this document - Thomas Graichen
and Seth Mos.
The SGI XFS project page
http://oss.sgi.com/projects/xfs/
is the definitive reference.
It contains pointers to whitepapers, books, articles, etc.
You could also join the xfs mailing list,
and there is also the #xfs IRC channel on irc.freenode.net.
Andreas Gruenbacher maintains the Extended Attribute and POSIX ACL
documentation for Linux at
http://acl.bestbits.at/
The acl(5) manual page is also quite extensive.
Linux native filesystem (83).
There are a number of mount options influencing XFS filesystems -
refer to the mount(8) manual page or the documentation in the
kernel source tree itself (Documentation/filesystems/xfs.txt)
No, there is no relation.
Newer utilities tend to mainly have fixes and checks the previous versions
might not have.
New features are also added in a backward compatible way - if they are
enabled via mkfs, an incapable (old) kernel will recognize that it does
not understand the new feature, and refuse to mount the filesystem.
XFS runs on all of the platforms that Linux supports.
It is more tested on the more common platforms, especially the i386 family.
Its also well tested on the IA64 platform since thats the platform SGI
Linux products use.
Yes.
To use quotas with XFS, you need to enable XFS quota support when you
configure your kernel. You also need to specify quota support when mounting.
You can get the Linux quota utilities at their sourceforge website
http://sourceforge.net/projects/linuxquota/ or use xfs_quota(8).
xfsdump(8) and xfsrestore(8) are fully supported.
The tape format is the same as on IRIX, so tapes are interchangeable
between operating systems.
This depends on where you install LILO.
Yes, for MBR (Master Boot Record) installations.
No, for root partition installations because the XFS superblock is
written at block zero, where LILO would be installed.
This is to maintain compatibility with the IRIX on-disk format, and
will not be changed.
There is native XFS filesystem support for GRUB starting with
version 0.91 and onward.
Unfortunately, GRUB used to make incorrect assumptions about being
able to read a block device image while a filesystem is mounted
and actively being written to, which could cause intermittent problems
when using XFS.
This has reportedly since been fixed, and the 0.97 version (at least)
of GRUB is apparently stable.
Yes.
Yes. The on-disk format of XFS is the same on IRIX and Linux. Obviously,
you should back-up your data before trying to move it between systems.
Filesystems must be "clean" when moved (i.e. unmounted).
If you plan to use IRIX filesystems on Linux keep the following points in mind:
the kernel needs to have SGI partition support enabled;
there is no XLV support in Linux, so you are unable to read IRIX filesystems
which use the XLV volume manager; also not all blocksizes available on
IRIX are available on Linux (only blocksizes less than or equal to the pagesize
of the architecture: 4k for i386, ppc, ... 8k for alpha, sparc, ... is
possible for now).
Make sure that the directory format is version 2 on the IRIX filesystems
(this is the default since IRIX 6.5.5).
Linux can only read v2 directories.
You can NOT make a XFS partition smaller online.
The only way to shrink is to do a complete dump, mkfs and restore.
An XFS filesystem may be enlarged by using
xfs_growfs(8).
If using partitions, you need to have free space after this partition to do so.
Remove partition, recreate it larger with the exact same starting
point.
Run xfs_growfs to make the partition larger.
Note - editing partition tables is a dangerous pastime, so
back up your filesystem before doing so.
Using XFS filesystems on top of a volume manager makes this a lot easier.
Things to include are what version of XFS you are using, if this is
a CVS version of what date and version of the kernel.
If you have problems with userland packages please report the version of the
package you are using.
If the problem relates to a particular filesystem, the output from the
xfs_info(8) command and any mount(8) options in use will
also be useful to the developers.
If you experience an oops, please run it through ksymoops so that
it can be interpreted.
If mount prints an error message something like:
you either do not have XFS compiled into the kernel (or you forgot
to load the modules) or you did not use the "-t xfs" option on mount
or the "xfs" option in /etc/fstab.
If you get something like:
Where can I find this FAQ?
Where can I find information about XFS?
Where can I find information about ACLs?
What partition type should I use for XFS?
What mount options does XFS have?
Is there any relation between the XFS utilities and the kernel version?
Does it run on platforms other than i386?
Do quotas work on XFS?
Are there any dump/restore tools for XFS?
Does LILO work with XFS?
Does GRUB work with XFS?
Can XFS be used for a root filesystem?
Will I be able to use IRIX XFS filesystems on Linux?
Is there a way to make a XFS filesystem larger or smaller?
What info should I include when reporting a problem?
Mounting a XFS filesystem does not work - what is wrong?
Does the filesystem have a undelete function?
How can I backup a XFS filesystem and ACLs?
I see applications returning error 990, what is wrong?
Why do I see binary NULLS in my files after recovery when I unplugged the power?
What is the problem with the write cache on journaled filesystems?
How can I tell if I have the write cache enabled?
How can I address the problem with the write cache?
Should barriers be enabled with storage which
has a persistent write cache?
What is the issue with directory corruption in Linux 2.6.17?
Q: Where can I find this FAQ?
Q: Where can I find documentation about XFS?
Q: Where can I find documentation about ACLs?
Q: What partition type should I use for XFS on Linux?
Q: What mount options does XFS have?
Q: Is there any relation between the XFS utilities and the kernel version?
Q: Does it run on platforms other than i386?
Q: Do quotas work on XFS?
Q: Are there any dump/restore tools for XFS?
Q: Does LILO work with XFS?
Q: Does GRUB work with XFS?
Q: Can XFS be used for a root filesystem?
Q: Will I be able to use my IRIX XFS filesystems on Linux?
Q: Is there a way to make a XFS filesystem larger or smaller?
Q: What information should I include when reporting a problem?
Q: Mounting a XFS filesystem does not work - what is wrong?
mount: /dev/hda5 has wrong major or minor number
mount: wrong fs type, bad option, bad superblock on /dev/sda1,
or too many mounted file systems
Refer to your system log file (/var/log/messages) for a
detailed diagnostic message from the kernel.
There is no undelete in XFS. Always keep backups.
You can backup a XFS filesystem with utilities like xfsdump(8) and standard tar(1) for standard files. If you want to backup ACLs you will need to use xfsdump, this is the only tool at the moment that supports backing up extended attributes. xfsdump can also be integrated with amanda(8).
The error 990 stands for EFSCORRUPTED which usually means XFS has detected a filesystem metadata problem and has shut the filesystem down to prevent further damage.
The cause can be pretty much anything, unfortunately - filesystem, virtual memory manager, volume manager, device driver, or hardware.
There should be a detailed console message when this initially happens. The messages have important information giving hints to developers as to the earliest point that a problem was detected. It is there to protect your data.
XFS journals metadata updates, not data updates. After a crash you are supposed to get a consistent filesystem which looks like the state sometime shortly before the crash, NOT what the in memory image looked like the instant before the crash.
Since XFS does not write data out immediately unless you tell it to with fsync, an O_SYNC or O_DIRECT open (the same is true of other filesystems), you are looking at an inode which was flushed out, but whose data was not. Typically you'll find that the inode is not taking any space since all it has is a size but no extents allocated (try examining the file with the xfs_bmap(8) command).
Many drives use a write back cache in order to speed up the performance of writes. However, there are conditions such as power failure when the write cache memory is never flushed to the actual disk. This causes problems for XFS and journaled filesystems in general because they rely on knowing when a write has completed to the disk. They need to know that the log information has made it to disk before allowing metadata to go to disk. When the metadata makes it to disk then the transaction can effectively be deleted from the log resulting in movement of the tail of the log and thus freeing up some log space. So if the writes never make it to the physical disk, then the ordering is violated and the log and metadata can be lost, resulting in filesystem corruption.
For SCSI/SATA:
For PATA/SATA (although for SATA this only works on a recent kernel with ATA command passthrough):
This disabling is kept persistent for a SCSI disk.
However, for a SATA/PATA disk this needs to be done after every reset as
it will reset back to the default of the write cache enabled.
And a reset can happen after reboot or on error recovery of the drive.
This makes it rather difficult to guarantee that the write cache is
maintained as disabled.
Some people have considered the idea of using an external log on a separate drive with the write cache disabled and the rest of the file system on another disk with the write cache enabled. However, that will not solve the problem. For example, the tail of the log is moved when we are notified that a metadata write is completed to disk and we won't be able to guarantee that if the metadata is on a drive with the write cache enabled.
In fact using an external log, will disable XFS' write barrier support.
Write barrier support is enabled by default in XFS since 2.6.17. It is disabled by mounting the filesystem with "nobarrier". Barrier support will flush the write back cache at the appropriate times (such as on XFS log writes). This is generally the recommended solution, however, you should check the system logs to ensure it was successful. Barriers will be disabled and reported in the log if any of the 3 scenarios occurs:
Many hardware RAID have a persistent write cache which preserves it across power failure, interface resets, system crashes, etc. Using write barriers in this instance is not warranted and will in fact lower performance. Therefore, it is recommended to turn off the barrier support and mount the filesystem with "nobarrier".
In the Linux kernel 2.6.17 release a subtle bug was accidentally introduced into the XFS directory code by some "sparse" endian annotations. This bug was sufficiently uncommon (it only affects a certain type of format change, in Node or B-Tree format directories, and only in certain situations) that it was not detected during our regular regression testing, but it has been observed in the wild by a number of people now.
Update: the fix is included in 2.6.17.7 and later kernels.
To add insult to injury, xfs_repair(8) is currently not correcting these directories on detection of this corrupt state either. This xfs_repair issue is actively being worked on, and a fixed version will be available shortly.
Update: a fixed xfs_repair is now available; version 2.8.10 or later of the xfsprogs package contains the fixed version.
No other kernel versions are affected. However, using a corrupt filesystem on other kernels can still result in the filesystem being shutdown if the problem has not been rectified (on disk), making it seem like other kernels are affected.
The xfs_check tool, or xfs_repair -n, should be able to detect any directory corruption.
Until a fixed xfs_repair binary is available, one can make use of the xfs_db(8) command to mark the problem directory for removal (see the example below). A subsequent xfs_repair invocation will remove the directory and move all contents into "lost+found", named by inode number (see second example on how to map inode number to directory entry name, which needs to be done _before_ removing the directory itself). The inode number of the corrupt directory is included in the shutdown report issued by the kernel on detection of directory corruption. Using that inode number, this is how one would ensure it is removed:
# xfs_db -x /dev/sdXXX xfs_db> inode NNN xfs_db> print core.magic = 0x494e core.mode = 040755 core.version = 2 core.format = 3 (btree) ... xfs_db> write core.mode 0 xfs_db> quit
A subsequent xfs_repair will clear the directory, and add new entries (named by inode number) in lost+found.
The easiest way to map inode numbers to full paths is via xfs_ncheck(8):
# xfs_ncheck -i 14101 -i 14102 /dev/sdXXX 14101 full/path/mumble_fratz_foo_bar_1495 14102 full/path/mumble_fratz_foo_bar_1494
Should this not work, we can manually map inode numbers in B-Tree format directory by taking the following steps:
# xfs_db -x /dev/sdXXX xfs_db> inode NNN xfs_db> print core.magic = 0x494e ... next_unlinked = null u.bmbt.level = 1 u.bmbt.numrecs = 1 u.bmbt.keys[1] = [startoff] 1:[0] u.bmbt.ptrs[1] = 1:3628 xfs_db> fsblock 3628 xfs_db> type bmapbtd xfs_db> print magic = 0x424d4150 level = 0 numrecs = 19 leftsib = null rightsib = null recs[1-19] = [startoff,startblock,blockcount,extentflag] 1:[0,3088,4,0] 2:[4,3128,8,0] 3:[12,3308,4,0] 4:[16,3360,4,0] 5:[20,3496,8,0] 6:[28,3552,8,0] 7:[36,3624,4,0] 8:[40,3633,4,0] 9:[44,3688,8,0] 10:[52,3744,4,0] 11:[56,3784,8,0] 12:[64,3840,8,0] 13:[72,3896,4,0] 14:[33554432,3092,4,0] 15:[33554436,3488,8,0] 16:[33554444,3629,4,0] 17:[33554448,3748,4,0] 18:[33554452,3900,4,0] 19:[67108864,3364,4,0]
At this point we are looking at the extents that hold all of the directory information. There are three types of extent here, we have the data blocks (extents 1 through 13 above), then the leaf blocks (extents 14 through 18), then the freelist blocks (extent 19 above). The jumps in the first field (start offset) indicate our progression through each of the three types. For recovering file names, we are only interested in the data blocks, so we can now feed those offset numbers into the xfs_db dblock command. So, for the fifth extent - 5:[20,3496,8,0] - listed above:
... xfs_db> dblock 20 xfs_db> print dhdr.magic = 0x58443244 dhdr.bestfree[0].offset = 0 dhdr.bestfree[0].length = 0 dhdr.bestfree[1].offset = 0 dhdr.bestfree[1].length = 0 dhdr.bestfree[2].offset = 0 dhdr.bestfree[2].length = 0 du[0].inumber = 13937 du[0].namelen = 25 du[0].name = "mumble_fratz_foo_bar_1595" du[0].tag = 0x10 du[1].inumber = 13938 du[1].namelen = 25 du[1].name = "mumble_fratz_foo_bar_1594" du[1].tag = 0x38 ...
So, here we can see that inode number 13938 matches up with name "mumble_fratz_foo_bar_1594". Iterate through all the extents, and extract all the name-to-inode-number mappings you can, as these will be useful when looking at "lost+found" (once xfs_repair has removed the corrupt directory).
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