Editing XFS FAQ

Info from: [http://oss.sgi.com/projects/xfs/faq.html main XFS faq at SGI]


Many thanks to earlier maintainers of this document - Thomas Graichen and Seth Mos.

== Q: Where can I find documentation about XFS? ==

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_email_list_and_archives|XFS mailing list]] or the '''<nowiki>#xfs</nowiki>''' IRC channel on ''irc.freenode.net''.

== Q: Where can I find documentation about ACLs? ==

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.

== Q: Where can I find information about the internals of XFS? ==

An [http://oss.sgi.com/projects/xfs/training/ SGI XFS Training course] aimed at developers, triage and support staff, and serious users has been in development. Parts of the course are clearly still incomplete, but there is enough content to be useful to a broad range of users.

Barry Naujok has documented the [http://oss.sgi.com/projects/xfs/papers/xfs_filesystem_structure.pdf XFS ondisk format] which is a very useful reference.

== Q: What partition type should I use for XFS on Linux? ==

Linux native filesystem (83).

== Q: What mount options does XFS have? ==

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 ([http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=Documentation/filesystems/xfs.txt;hb=HEAD Documentation/filesystems/xfs.txt])

== Q: Is there any relation between the XFS utilities and the kernel version? ==

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.

== Q: Does it run on platforms other than i386? ==

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.

== Q: Quota: Do quotas work on XFS? ==

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/  http://sourceforge.net/projects/linuxquota/] or use '''xfs_quota(8)'''.

== Q: Quota: What's project quota? ==

The  project  quota  is a quota mechanism in XFS can be used to implement a form of directory tree quota, where a specified directory and all of the files and subdirectories below it (i.e. a tree) can be restricted to using a subset of the available space in the filesystem.

== Q: Quota: Can group quota and project quota be used at the same time? ==

No, project quota cannot be used with group quota at the same time. On the other hand user quota and project quota can be used simultaneously.

== Q: Quota: Is umounting prjquota (project quota) enabled fs and mounting it again with grpquota (group quota) removing prjquota limits previously set from fs (and vice versa) ? ==

To be answered.

== Q: Are there any dump/restore tools for XFS? ==

'''xfsdump(8)''' and '''xfsrestore(8)''' are fully supported. The tape format is the same as on IRIX, so tapes are interchangeable between operating systems.

== Q: Does LILO work with XFS? ==

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.

== Q: Does GRUB work with XFS? ==

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.

== Q: Can XFS be used for a root filesystem? ==

Yes, with one caveat: Linux does not support an external XFS journal for the root filesystem via the "rootflags=" kernel parameter. To use an external journal for the root filesystem in Linux, an init ramdisk must mount the root filesystem with explicit "logdev=" specified. [http://gus3.typepad.com/i_am_therefore_i_think/2008/07/scratching-an-i.html More information here.]

== Q: Will I be able to use my IRIX XFS filesystems on Linux? ==

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.

== Q: Is there a way to make a XFS filesystem larger or smaller? ==

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.

== Q: What information should I include when reporting a problem? ==

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 you have a filesystem that cannot be repaired, make sure you have xfsprogs 2.9.0 or later and run '''xfs_metadump(8)''' to capture the metadata (which obfuscates filenames and attributes to protect your privacy) and make the dump available for someone to analyse.

== Q: Mounting an XFS filesystem does not work - what is wrong? ==

If mount prints an error message something like:


     mount: /dev/hda5 has wrong major or minor number

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 <tt>/etc/fstab</tt>.

If you get something like:


 mount: wrong fs type, bad option, bad superblock on /dev/sda1,
        or too many mounted file systems

Refer to your system log file (<tt>/var/log/messages</tt>) for a detailed diagnostic message from the kernel.

== Q: Does the filesystem have an undelete capability? ==

There is no undelete in XFS. However at least some XFS driver implementations does not wipe file information nodes completely so there are chance to recover files with specialized commercial software like [http://www.ufsexplorer.com/rdr_xfs.php Raise Data Recovery for XFS].
In this kind of XFS driver implementation it does not re-use directory entries immediately so there are chance to get back recently deleted files even with their real names.

This applies to most recent Linux distributions, as well as to most popular NAS boxes that use embedded linux and XFS file system.

Anyway, the best is to always keep backups.

== Q: How can I backup a XFS filesystem and ACLs? ==

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''' or [http://www.bacula.org/en/dev-manual/Current_State_Bacula.html Bacula] (> version 3.1.4) or [http://rsync.samba.org/ rsync] (>= version 3.0.0) to backup ACLs and EAs. '''xfsdump''' can also be integrated with [http://www.amanda.org/ amanda(8)].

== Q: I see applications returning error 990 or "Structure needs cleaning", what is wrong? ==

The error 990 stands for [http://oss.sgi.com/cgi-bin/gitweb.cgi?p=xfs/xfs.git;a=blob;f=fs/xfs/linux-2.6/xfs_linux.h#l145 EFSCORRUPTED] which usually means XFS has detected a filesystem metadata problem and has shut the filesystem down to prevent further damage. Also, since about June 2006, we [http://oss.sgi.com/cgi-bin/gitweb.cgi?p=xfs/xfs.git;a=commit;h=da2f4d679c8070ba5b6a920281e495917b293aa0 converted from EFSCORRUPTED/990 over to using EUCLEAN], "Structure needs cleaning."

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.

You can use xfs_check and xfs_repair to remedy the problem (with the file system unmounted).

== Q: Why do I see binary NULLS in some files after recovery when I unplugged the power? ==

Update: This issue has been addressed with a CVS fix on the 29th March 2007 and merged into mainline on 8th May 2007 for 2.6.22-rc1.

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).

== Q: What is the problem with the write cache on journaled filesystems? ==

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.  Further, the drive can de-stage data from the write cache to the platters in any order that it chooses.  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.

With hard disk cache sizes of currently (Jan 2009) up to 32MB that can be a lot of valuable information.  In a RAID with 8 such disks these adds to 256MB, and the chance of having filesystem metadata in the cache is so high that you have a very high chance of big data losses on a power outage.

With a single hard disk and barriers turned on (on=default), the drive write cache is flushed before and after a barrier is issued.  A powerfail "only" loses data in the cache but no essential ordering is violated, and corruption will not occur.

With a RAID controller with battery backed controller cache and cache in write back mode, you should turn off barriers - they are unnecessary in this case, and if the controller honors the cache flushes, it will be harmful to performance.  But then you *must* disable the individual hard disk write cache in order to ensure to keep the filesystem intact after a power failure. The method for doing this is different for each RAID controller. See the section about RAID controllers below.

== Q: How can I tell if I have the disk write cache enabled? ==

For SCSI/SATA:

* Look in dmesg(8) output for a driver line, such as:<br /> "SCSI device sda: drive cache: write back"
* <nowiki># sginfo -c /dev/sda | grep -i 'write cache' </nowiki>

For PATA/SATA (although for SATA this only works on a recent kernel with ATA command passthrough):

* <nowiki># hdparm -I /dev/sda</nowiki><br /> and look under "Enabled Supported" for "Write cache"

For RAID controllers:

* See the section about RAID controllers below

== Q: How can I address the problem with the disk write cache? ==

=== Disabling the disk write back cache. ===

For SATA/PATA(IDE): (although for SATA this only works on a recent kernel with ATA command passthrough):<br />

* <nowiki># hdparm -W0 /dev/sda</nowiki><br /> # hdparm -W0 /dev/hda
* <nowiki># blktool /dev/sda wcache off</nowiki><br /> # blktool /dev/hda wcache off

For SCSI:

* Using sginfo(8) which is a little tedious<br /> It takes 3 steps. For example:
*# <nowiki>#sginfo -c /dev/sda</nowiki><br /> which gives a list of attribute names and values
*# <nowiki>#sginfo -cX /dev/sda</nowiki><br /> which gives an array of cache values which you must match up with from step 1, e.g.<br /> 0 0 0 1 0 1 0 0 0 0 65535 0 65535 65535 1 0 0 0 3 0 0
*# <nowiki>#sginfo -cXR /dev/sda 0 0 0 1 0 0 0 0 0 0 65535 0 65535 65535 1 0 0 0 3 0 0</nowiki><br /> allows you to reset the value of the cache attributes.

For RAID controllers:

* See the section about RAID controllers below

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.<br />

=== Using an external log. ===

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. ===

Write barrier support is enabled by default in XFS since kernel version 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:

* "Disabling barriers, not supported with external log device"
* "Disabling barriers, not supported by the underlying device"
* "Disabling barriers, trial barrier write failed"

If the filesystem is mounted with an external log device then we currently don't support flushing to the data and log devices (this may change in the future). If the driver tells the block layer that the device does not support write cache flushing with the write cache enabled then it will report that the device doesn't support it. And finally we will actually test out a barrier write on the superblock and test its error state afterwards, reporting if it fails.

== Q. Should barriers be enabled with storage which has a persistent write cache? ==

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 recommended and will in fact lower performance. Therefore, it is recommended to turn off the barrier support and mount the filesystem with "nobarrier". But take care about the hard disk write cache, which should be off.

== Q. Which settings does my RAID controller need ? ==

It's hard to tell because there are so many controllers. Please consult your RAID controller documentation to determine how to change these settings, but we try to give an overview here:

Real RAID controllers (not those found onboard of mainboards) normally have a battery backed cache (or an [http://en.wikipedia.org/wiki/Electric_double-layer_capacitor ultracapacitor] + flash memory "[http://www.tweaktown.com/articles/2800/adaptec_zero_maintenance_cache_protection_explained/ zero maintenance cache]") which is used for buffering writes to improve speed. Even if it's battery backed, the individual hard disk write caches need to be turned off, as they are not protected from a powerfail and will just lose all contents in that case.

* onboard RAID controllers: there are so many different types it's hard to tell. Generally, those controllers have no cache, but let the hard disk write cache on. That can lead to the bad situation that after a powerfail with RAID-1 when only parts of the disk cache have been written, the controller doesn't even see that the disks are out of sync, as the disks can resort cached blocks and might have saved the superblock info, but then lost different data contents. So, turn off disk write caches before using the RAID function.

* 3ware: /cX/uX set cache=off, see http://www.3ware.com/support/UserDocs/CLIGuide-9.5.1.1.pdf , page 86

* Adaptec: allows setting individual drives cache
arcconf setcache <disk> wb|wt
wb=write back, which means write cache on, wt=write through, which means write cache off. So "wt" should be chosen.

* Areca: In archttp under "System Controls" -> "System Configuration" there's the option "Disk Write Cache Mode" (defaults "Auto")

"Off": disk write cache is turned off

"On": disk write cache is enabled, this is not save for your data but fast

"Auto": If you use a BBM (battery backup module, which you really should use if you care about your data), the controller automatically turns disk writes off, to protect your data. In case no BBM is attached, the controller switches to "On", because neither controller cache nor disk cache is save so you don't seem to care about your data and just want high speed (which you get then).

That's a very sensible default so you can let it "Auto" or enforce "Off" to be sure.

* LSI MegaRAID: allows setting individual disks cache:
MegaCli -AdpCacheFlush -aN|-a0,1,2|-aALL -EnDskCache|DisDskCache

* Xyratex: from the docs: "Write cache includes the disk drive cache and controller cache.". So that means you can only set the drive caches and the unit caches together. To protect your data, turn it off, but write performance will suffer badly as also the controller write cache is disabled.

== Q: Which settings are best with virtualization like VMware, XEN, qemu? ==

The biggest problem is that those products seem to also virtualize disk 
writes in a way that even barriers don't work anymore, which means even 
a fsync is not reliable. Tests confirm that unplugging the power from 
such a system even with RAID controller with battery backed cache and 
hard disk cache turned off (which is save on a normal host) you can 
destroy a database within the virtual machine (client, domU whatever you 
call it).

In qemu you can specify cache=off on the line specifying the virtual 
disk. For others information is missing.

== Q: What is the issue with directory corruption in Linux 2.6.17? ==

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:

 <nowiki>
 # xfs_db -x /dev/sdXXX
 xfs_db&gt; inode NNN
 xfs_db&gt; print
 core.magic = 0x494e
 core.mode = 040755
 core.version = 2
 core.format = 3 (btree)
 ...
 xfs_db&gt; write core.mode 0
 xfs_db&gt; quit
 </nowiki>

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)'''<nowiki>: </nowiki>

 <nowiki>
 # xfs_ncheck -i 14101 -i 14102 /dev/sdXXX
       14101 full/path/mumble_fratz_foo_bar_1495
       14102 full/path/mumble_fratz_foo_bar_1494
 </nowiki>

Should this not work, we can manually map inode numbers in B-Tree format directory by taking the following steps:

 <nowiki>
 # xfs_db -x /dev/sdXXX
 xfs_db&gt; inode NNN
 xfs_db&gt; 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&gt; fsblock 3628
 xfs_db&gt; type bmapbtd
 xfs_db&gt; 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]
 </nowiki>

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&gt; dblock 20
 xfs_db&gt; 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).


== Q: Why does my > 2TB XFS partition disappear when I reboot ? ==

Strictly speaking this is not an XFS problem.

To support > 2TB partitions you need two things: a kernel that supports large block devices (<tt>CONFIG_LBD=y</tt>) and a partition table format that can hold large partitions.  The default DOS partition tables don't.  The best partition format for
> 2TB partitions is the EFI GPT format (<tt>CONFIG_EFI_PARTITION=y</tt>).

Without CONFIG_LBD=y you can't even create the filesystem, but without <tt>CONFIG_EFI_PARTITION=y</tt> it works fine until you reboot at which point the partition will disappear.  Note that you need to enable the <tt>CONFIG_PARTITION_ADVANCED</tt> option before you can set <tt>CONFIG_EFI_PARTITION=y</tt>.

== Q: Why do I receive <tt>No space left on device</tt> after <tt>xfs_growfs</tt>? ==

After [http://oss.sgi.com/pipermail/xfs/2009-January/039828.html growing a XFS filesystem], df(1) would show enough free space but attempts to write to the filesystem result in -ENOSPC. To fix this, [http://oss.sgi.com/pipermail/xfs/2009-January/039835.html Dave Chinner advised]:

  The only way to fix this is to move data around to free up space
  below 1TB. Find your oldest data (i.e. that was around before even
  the first grow) and move it off the filesystem (move, not copy).
  Then if you copy it back on, the data blocks will end up above 1TB
  and that should leave you with plenty of space for inodes below 1TB.
  
  A complete dump and restore will also fix the problem ;)

Also, you can add 'inode64' to your mount options to allow inodes to live above 1TB.

== Q: Is using noatime or/and nodiratime at mount time giving any performance benefits in xfs (or not using them performance decrease)? ==

See: [http://everything2.com/title/Filesystem+performance+tweaking+with+XFS+on+Linux Filesystem performance tweaking with XFS on Linux]

== Q: How to get around a bad inode repair is unable to clean up ==

The trick is go in with xfs_db and mark the inode as a deleted, which will cause repair to clean it up and finish the remove process.

  xfs_db -x -c 'inode XXX' -c 'write core.nextents 0' -c 'write core.size 0' /dev/hdXX

== Q: How to calculate the correct sunit,swidth values for optimal performance ==

XFS allows to optimize for a given RAID stripe size and number of disks via mount options. The calculation of these values is quite simple:

  su = <RAID controllers stripe size in BYTES (or KiBytes when used with k)>
  sw = <# of data disks>

So if your RAID controller has a stripe size of 64KB, and you have a RAID-6 with 8 disks, use

  su = 64k
  sw = 6 (RAID-6 of 8 disks has 6 data disks)

A RAID stripe size of 256KB with a RAID-10 over 16 disks should use

  su = 256k
  sw = 8 (RAID-10 of 16 disks has 8 data disks)

Alternatively, you can use "sunit" instead of "su", but then the value means "number of 512B sectors", and you *must* use "swidth" then as "number of 512B sectors" then as well!

Please be aware that when xfs_info or mkfs.xfs report the sunit/swidth values, they use a different unit size than above.  Whereas sunit and swidth are first specified in 512B sectors, xfs_info and mkfs.xfs report them in multiples of your basic block size.

You can check this quite easily.  Assuming a swidth/sunit of 1024/4096, and a block size of 4096, you should see a reported sunit/swidth of 128/512.  1024 * 512 == 128 * 4096.

== Q: Why doesn't NFS-exporting subdirectories of inode64-mounted filesystem work? ==

The default <tt>fsid</tt> type encodes only 32-bit of the inode number for subdirectory exports.  However, exporting the root of the filesystem works, or using one of the non-default <tt>fsid</tt> types (<tt>fsid=uuid</tt> in <tt>/etc/exports</tt> with recent <tt>nfs-utils</tt>) should work as well. (Thanks, Christoph!)

== Q: What is the inode64 mount option for? ==

By default, with 32bit inodes, XFS places inodes only in the first 1TB of a disk. If you have a disk with 100TB, all inodes will be stuck in the first TB. This can lead to strange things like "disk full" when you still have plenty space free, but there's no more place in the first TB to create a new inode. Also, performance sucks.

To come around this, use the inode64 mount options for filesystems >1TB. Inodes will then be placed in the location where their data is, minimizing disk seeks.

Beware that some old programs might have problems reading 64bit inodes, especially over NFS. Your editor used inode64 for over a year with recent (openSUSE 11.1 and higher) distributions using NFS and Samba without any corruptions, so that might be a recent enough distro.

== Q: Can I just try the inode64 option to see if it helps me? ==

Starting from kernel 2.6.35, you can try and then switch back. Older kernels have a bug leading to strange problems if you mount without inode64 again. For example, you can't access files & dirs that have been created with an inode >32bit anymore.

== Q: Performance: mkfs.xfs -n size=64k option ==

Asking the implications of that mkfs option on the XFS mailing list, Dave Chinner explained it this way:

Inodes are not stored in the directory structure, only the directory entry name and the inode number. Hence the amount of space used by a
directory entry is determined by the length of the name.

There is extra overhead to allocate large directory blocks (16 pages instead of one, to begin with, then there's the vmap overhead, etc), so for small directories smaller block sizes are faster for create and unlink operations.

For empty directorys, operations on 4k block sized directories consume roughly 50% less CPU that 64k block size directories. The 4k block size directories consume less CPU out to roughly 1.5 million entries where the two are roughly equal. At directory sizes of 10 million entries, 64k directory block operations are consuming about 15% of the CPU that 4k directory block operations consume.

In terms of lookups, the 64k block directory will take less IO but consume more CPU for a given lookup. Hence it depends on your IO latency and whether directory readahead can hide that latency as to which will be faster. e.g. For SSDs, CPU usage might be the limiting factor, not the IO. Right now I don't have any numbers on what the difference might be - I'm getting 1 billion inode population issues worked out first before I start on measuring cold cache lookup times on 1 billion files....