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<FONT FACE="ARIAL NARROW, HELVETICA" SIZE="5"><B>Linux XFS FAQ</B></FONT>
<FONT FACE="ARIAL NARROW, HELVETICA">
<h2>
Q: Where can I find this FAQ?
</h2>
<P>
Currently at:
</P>
<ul>
    <A HREF="http://oss.sgi.com/projects/xfs/faq.html">http://oss.sgi.com/projects/xfs/faq.html</A>
</ul>
<P>
If you have any comments or suggestions just let me know:
</P>
<ul>
    <A HREF="mailto:tgr@spoiled.org">tgr@spoiled.org</A>
</ul>
<h2>
Q: What is XFS?
</h2>
<P>
XFS is a journalling filesystem developed by SGI and used in SGI's
IRIX operating system. It is now also available under GPL for linux. 
It is extremely scalable, using btrees extensively to support large
and/or sparse files, and extremely large directories. The journalling
capability means no more waiting for fsck's or worrying about meta-data
corruption.
</P>
<h2>
Q: Where can I find information about XFS for linux?
</h2>
<P>
Just point your web-browser to:
</P>
<ul>
    <A HREF="http://oss.sgi.com/projects/xfs/">http://oss.sgi.com/projects/xfs/</A>
</ul>
<h2>
Q: What do I need to build an XFS ready kernel?
</h2>
<P>
The best way to do this is to checkout the SGI XFS kernel from
their CVS tree. How to do this is described at
</P>
<ul>
    <A HREF="http://oss.sgi.com/projects/xfs/cvs_download.html">http://oss.sgi.com/projects/xfs/cvs_download.html</A>
</ul>
<P>
After that you have two subtrees of importance: linux and cmd. The
firsts one - linux - is a normal linux kernel source tree containing
the XFS code. It is updated to the latest available linux kernel
from time to time but it may be a bit (not much) behind. Just build
your kernel the way you are used to do it and don't forget to enable
XFS and pagebuf under filesystems. You also need to enable the
ask for experimental features option in the "Code maturity level
options" menu. The other tree - cmd - contains all the tools you need
- most important: <tt>mkfs.xfs</tt> and <tt>xfs_repair</tt> - just go
to the xfs subdirectory in cmd and type make (maybe followed by make
install to install the binaries). The cmd tree has been restructured
a bit at the beginning of 2001: now you have to go into the various
subdirs (xfsprogs, attr, acl, xfsdump, dmapi) and run the make or the
below described Makepkgs script there. One thing which is also important
to note is that you need to have the e2fs-devel package installed in
order to build the cmd tools - because they require the uuid stuff in
them. You may also build src and binary rpms by running:
</P>
<pre>
    ./Makepkgs verbose
</pre>
<P>
in the cmd/xfs or cmd/[xfsprogs, attr, ...] directory.  The tools also
have man-pages which you may consult for interesting options.
</P>
<P>
There exists also another way to get an XFS ready kernel - you may
get kernel patches relative to a official kernel from:
</P>
<ul>
    <A HREF="ftp://oss.sgi.com/projects/xfs/download">ftp://oss.sgi.com/projects/xfs/download</A>
</ul>
<P>
and apply then to the kernel sources the patch is for. This is a
good way for all the people who don't want to use CVS or do not
have the bandwidth to checkout the whole kernel tree.
</P>
<P>
A third way to get a XFS ready system and kernel is to use the
prepared rpm's and experimental redhat install ISO images from
SGI you may find also in the download area. Also Bryan J. Smith
is building binary rpm's of XFS enabled kernels and the tools
for RedHat which can be found at
</P>
<P>
<ul>
    <A HREF="http://www.smithconcepts.com/files/xfs/">http://www.smithconcepts.com/files/xfs/</A>
</ul>
</P>
<h2>
Q: How do I use XFS?
</h2>
<P>
There you will also find a little HOWTO about the nessecary steps.
</P>
<P>
Just reboot the new build kernel and create a filesytem on an empty
partition
</P>
<pre>
    mkfs -t xfs /dev/foo
</pre>
<P>
where /dev/foo is the the partition you want to use (you may have
to use the -f option of <tt>mkfs -t xfs</tt> (which calls mkfs.xfs from
the cmd directory which you must have installed) if this partition already
contains an old filesystem which you want to overwrite). Now you can
mount the filesystem using:
</P>
<pre>
    mount -t xfs /dev/foo /somewhere
</pre>
<h2>
Q: How stable is XFS?
</h2>
<P>
It is quite useable now but I would not recommend it for production
use right now - but I think it will be there quite soon. I used
it here in a test scenario for a 50 people squid and a ~20 mb/day
newsfeed with 30 simulated users on an XFS only machine for quite some
time and had no problems so far. Also SGI is doing quite a lot of
stress tests with it (see the stress subdir of the cmd tree). In the
last weeks and months more and more people are evaluating XFS in
production-like environments with mostly good results. At SGI it gets
regular tested on a 4 cpu box and the server hosting the linux XFS
project web-pages is also completely running on XFS.
</P>
<h2>
Q: Does it run on platforms other than i386?
</h2>
<P>
The current XFS tree seems to work just fine on ppc now (aside from some
trivial compile fixes). It also runs well and is getting sporadically
tested on the alpha, sparc64 and ia64. But on all of those platforms it is
not as well tested as on i386, but so far there are no major problems
on those platforms known.
All in all it looks like XFS will be running across a lot of platforms
fine soon (with all the platforms above we have 32/64bit and
little/big-endian architectures supported. If you run it on a platform
not mentioned here please let me know so that i can add it.
Also an important note is that XFS is inherently platform
independent in the on disk layout - so it should be possible to move
a XFS disk from one linux platform to another out of the box.
</P>
<P>
If you are interested in this you may have a look at
</P>
<ul>
    <A HREF="http://people.spoiled.org/tgr/unix/xfs/">http://people.spoiled.org/tgr/unix/xfs/</A>
</ul>
</P>
Especially the ppc subdir there contains mostly quite up to date
merged XFS and ppc trees which should work on ppc machines quite
fine.
<P>
<h2>
Q: Do quotas work on XFS?
</h2>
<P>
User quotas are supported, group/project quotas are work-in-progress. 
</p>
<p>To use quotas with XFS, you need to enable linux quota support, and
XFS quota support when you configure your kernel. You also need to specify 
quota support when you mount. A patched version of the linux quota tools 
which supports XFS is in the <tt>cmd/quota</tt> directory of the XFS
distribution. Hopefully XFS will be supported in the standard quota tools
soon.
</P>
<h2>
Q: Is there any dump/restore for XFS?
</h2>
<P>
xfsdump and xfsrestore are now in the CVS tree. The tape format is
the same as for xfsdump and xfsrestore on IRIX and dump tapes should
be interchangable between systems. 
</p>
<p>
The tape format is <em>not the same</em> as the classic Unix dump but should 
work fine with tools like Amanda. Dumps produced with standard 
other dump programs should be able to be restored onto an XFS filesystem 
using the coresponding restore program.
</P>
<h2>
Q: Does LILO work with XFS?
</h2>
<P>
This depens on where you install LILO.
For MBR installation:  Yes.
For root partitions: No, because the XFS superblock goes where LILO
would be installed.  This is to maintain compatibility with the Irix
on-disk format.
</P>
<h2>
Q: Does GRUB work with XFS?
</h2>
<P>
No - so far there is no direct support in GRUB to boot from an XFS
filesystem but you may still boot an mostly XFS system with a small
ext2 /boot partition with all the GRUB stuff on it using GRUB. Btw.
writing XFS support for GRUB would be a good way to learn more about
XFS if anyone is interested :-).
</P>
<h2>
Q: Can I run XFS on top of LVM?
</h2>
<P>
Yes you can but you need to use a patch for LVM because the XFS
kernel contains stephen tweedies sard patches which change the
format of /proc/partitions. Also for using XFS on LVM you should
not enable the kiobuf option in the kernel XFS compile options.
You may get a patch (created by William L Jones) to start from at
</P>
<ul>
    <A HREF="http://people.spoiled.org/tgr/unix/xfs/old/lvm.patch">http://people.spoiled.org/tgr/unix/xfs/old/lvm.patch</A>
</ul>
<P>
See the head of it for details and a list of contributors. With the
current lvm 0.9.1 betas it is reported to work now out of the box, but
there is still no kiobuf support for lvm and md devices. Also here
please always stay close to the current cvs version of the XFS tree.
</P>
<h2>
Q: Can I use XFS on loopback block devices?
</h2>
<P>
Yes. If you are using a 2.4.2 based XFS kernel you need to apply Jens
Axboes loop-xfs-7c fix for it to work (the fix is for a problem in
2.4.2 and has nothing really to do with XFS). You may get this patch from
</P>
<ul>
    <A HREF="ftp://ftp.kernel.org/pub/linux/kernel/people/axboe/xfs/">ftp://ftp.kernel.org/pub/linux/kernel/people/axboe/xfs/</A>
</ul>
<P>
</P>
<h2>
Q: Can XFS be used for a root filesystem?
</h2>
<P>
Yes. A document describing how to convert your system to XFS is currently
being worked on. The current state of this document you may find at
</P>
<ul>
    <A HREF="http://www.gargoylecc.com/Linux+XFS-HOWTO.html">http://www.gargoylecc.com/Linux+XFS-HOWTO.html</A>
</ul>
<P>
You might also have a look at my little stand-alone
environment for converting and maintaining a root XFS filesystem at
</P>
<ul>
    <A HREF="http://people.spoiled.org/tgr/unix/miniroot/">http://people.spoiled.org/tgr/unix/miniroot/</A>
</ul>
<P>
</P>
<h2>
Q: What partition type should I use for XFS? 
</h2>
<P>
Linux native filesystem (83).
</P>
<h2>
Q: Mounting the XFS filesystem does not work - what is wrong?
</h2>
<P>
If you get something like:
</P>
<pre>
    mount: /dev/hda5 has wrong major or minor number
</pre>
<P>
you either don't 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>.
</P>
<h2>
Q: Does XFS run with the linux software RAID md driver?
</h2>
<P>
It seems to work with a special patch from Russel Cattelan which
will be incorporated into the main tree soon (i think it is in now
since around march 13th).
Also keep in mind - if you want to try it - also the md driver
will definitely not work with the kiobuf option enabled.
</P>
<h2>
Q: Does XFS run on hardware RAID?
</h2>
<P>
Yes - hardware RAID is like a normal disk to XFS (like to any other
filesystem) and this should not be a problem.
</P>
<h2>
Q: Can I run NFS on top of an XFS partition?
</h2>
<P>
Yes. To get good performance make shure to use an XFS tree from after
mid-march 2001. There were some important fixes for useable NFS-performance.
So far there are no more known problems with XFS and NFS since then.
</P>
<h2>
Q: Is there a way to resize an XFS filesystem?
</h2>
<P>
An XFS filesystem may be enlarged within a partition using 
<tt>xfs_growfs</tt>.  Back up your filesystem before using this
tool.
</P>
<h2>
Q: Will I be able to use my old IRIX XFS disks on linux?
</h2>
<P>
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 (ie unmounted correctly).
</p>
<p>
The only real caveat here is that at the moment XFS on linux only supports
filesystems where the disk block size equals the page size. Support
for other block sizes available on IRIX is work-in-progress.
</P>
<h2>
Q: Is there documentation on how XFS works?
</h2>
<P>
There are some papers available on the SGI XFS project page (mentioned
above) and I have set up an LXR indexed version of the SGI XFS CVS tree
at:
</P>
<ul>
    <A HREF="http://innominate.org/~graichen/projects/lxr/source/?v=xfs">http://innominate.org/~graichen/projects/lxr/source?v=xfs</A>
</ul>
<P>
which might help to find an easy and good way through
the sources. I plan to keep this tree automatically updated to the
current SGI XFS CVS version on a daily basis within the next days.
If anyone has pointers to other XFS related docs - just send me a
mail (address - see above).
</P>
<h2>
Q: Processes are hanging when acting on an XFS filesystem (for
instance in sv_wait) - what is this?
</h2>
<p>
Recompile your kernel with gcc 2.91.66 instead of gcc 2.95 this should
fix your problem. On RedHat 7.x systems you get egcs via kgcc. See the
comments in the top level Makefile of the kernel for more information
about this.
</P>
<h2>
Q: Are there any known issues about gcc 2.95 for compiling the
XFS kernel tree?
</h2>
<p>
Yes. So far there were some problems reported with kernels built with
gcc 2.95 which were solved by compiling it with egcs 2.91.66 (or kgcc on
RedHat 7.x systems). So for now please
use version gcc 2.91.66 (aka egcs 1.1.2) to build your XFS kernel. If you
are using a debian or SuSE based system this means that you may have to
find and install this egcs version. Please note that the problems with
gcc 2.95 seem to be restriced to the i386 platform - on the ppc it works
just fine with 2.95 for instance. All said for gcc 2.95 also applies to
redhat's gcc 2.96. On the other hand the gcc 2.95.3 (20010125) from
debian unstalbe seems to work.

</P>
<h2>
Q: What are all those other things in the cmd/xfs directory?
</h2>
<p>
Some of them are other interesting tools like: db - <tt>xfs_db</tt> is a xfs
filesystem debugger (working) , copy - <tt>xfs_copy</tt> is tool for 
effectively copying one filesystem to another device (not yet ported), 
fsr - <tt>xfs_fsr</tt> is a defragmenter for xfs (working), 
repair - <tt>xfs_repair</tt> is the consistency checker for an xfs 
filesystem (working). As already mentioned earlier: the cmd structure
has changed a bit at the beginning of 2001 - now it all looks a bit
clearer i think (modeled a bit after the ext2 tools structure). The
only two other subdirs are: quota - a quota-tools tree with the XFS
additions in it and xfstests - the SGI XFS stress test suite.
</p>
<h2>
Q: Why my RedHat 7.0 system does not boot with the XFS beta kernel?
</h2>
<p>
This is because there is a syscall conflict which results in this
problem. Please use the latest XFS code from the cvs tree which has
this fixed. Eventually the beta will be rerolled for that.
</p>
<h2>
Q: My filesystem is ok - but xfs_check or xfs_repair shows errors - whats wrong here?
</h2>
<p>
Don't run those commands on a mounted filesystem - in this case it is
normal that you see inconsistencies. Never run xfs_repair on a mounted
filesystem without the "-n" switch at all. If you ran the commands on
an unmounted filesystem then there seems to be something wrong with
the filesystem.
</P>
<h2>
Q: I am getting very bad dbench (etc.) results with XFS - whats wrong here?
</h2>
<P>
First dbench: this is a very metadata intensive benchmark which might
be a little limited by the default log size. You will getter better
results by creating the filesystems with
</P>
<P>
<PRE>
    mkfs -t xfs -l size=32768b -f /dev/device
</PRE>
</P>
<P>
This creates a bigger logspace. Also it is a good idea to mount meta-data
intensive filesystems with
</P>
<P>
<PRE>
    mount -t xfs -o logbufs=8,logbsize=32768 /dev/device /mountpoint
</PRE>
</P>
<P>
Note that in some earlier versions you may only use 4 logbufs as a maximum.
Also since mid-march there are some changes in the tree which improve
the overall dbench performance a bit.
Have a look for the xfs.txt file in the Documentation/filesystems
subdirectory of your kernel sources for those and other XFS mount
options.
</P>
<P>
In general you should get about the same or better performance values
with XFS in various benchmarks. One thing XFS is usually bad at is
removing large amounts of files (rm -rf or bonnie++).
</P>
<h2>
Q: Mounting or umounting an XFS root filesystem takes a very long time - why?
</h2>
<P>
This is fixed in the current code (mid-march). But also before this is
normal and harmless - it only takes a bit of time (on boot and halt with
SuSE startup scripts - only on halt with RedHat based scripts).
</P>
<P>

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