[Dovecot] XFS vs EXT4 for mail storage

[Charles Marcus]

On 2013-05-03 1:30 AM, Stan Hoeppner <stan at hardwarefreak.com> wrote:
>  From a filesystem perspective mdbox is little different from maildir as
> they both exhibit lots of small random IOs.

Hi Stan. Thanks, was hoping you'd chime in here...

But, I'm confused as to why you'd say this. mdbox supposedly has many 
advantages over maildir, since it is *not* a single file for every email 
(like maildir or sdbox).

> My recommendation is to use XFS with the defaults, but add "inode64" to
> the mount options in /etc/fstab.  This enables the modern allocator
> which clusters files around their parent directory within an allocation
> group.  It's the default allocator in very recent upstream kernels but
> not in most currently shipping distro kernels.  It decreases seek
> latency between metadata and file operations, and better manages on disk
> space.  In short, XFS will yield superior mail performance to EXT4 in a
> multiuser environment.

Thanks very much. I'd already come to a similar conclusion, but was 
starting to have doubts after some of the prior comments. But what you 
say backs up the majority of what I've been reading. It's just difficult 
to judge what you're reading when you aren't a software or hardware 
engineer, just a lowly self-taught sysadmin who still consider himself a 
noob even after doing this for a few years.

> There are currently no mail workload tuning docs in the world of XFS
> that I'm aware of.  I've been intending to write such a doc for the
> XFS.org FAQ for some time but it hasn't happened yet.

Hope you find the time to do it some day... :)

On 2013-05-03 5:54 AM, Stan Hoeppner <stan at hardwarefreak.com> wrote:
> Many XFS mount options are kernel version specific.  Show:
> ~$ uname -a

Linux myhost 3.7.10-gentoo-r1 #3 SMP Sat Apr 27 10:01:59 EDT 2013 x86_64 
AMD Opteron(tm) Processor 4180 AuthenticAMD GNU/Linux

> Delaylog doesn't exist in recent mount(8) because it's no longer a mount
> option, same goes for older mount(8).  Its existence as a mount option
> didn't exist for long, WRT distro mount(8) updates.  Since 2.6.39
> delaylog is the default, and as of somewhat more recently in the 3.x
> tree, the old journal logging code was completely removed from the
> source.  Thus there is no longer a "delaylog" mount option.  The feature
> is now hard coded in XFS.

Got it, thanks.

> Barriers are crucial to XFS journal, and thus filesystem, reliability.
> "nobarrier" isn't listed in mount(8), though "barrier" is, which is the
> default mode.  Making people "look for" the switch that disables
> barriers forces them to take a learning journey.  Hopefully during this
> journey they become educated to the risks of disabling it, before doing
> so.  "Better reliability through obscurity" you might say.  Consider the
> horrible rap XFS would have today if everyone and his dog could easily
> learn how to disable barriers, then did so on hardware not appropriate
> for it.  Yes, exactly, corrupted XFS filesystems littering the landscape
> and people screaming what a pile of dogsh|zt XFS is.

Got it, thanks again.

> WRT noquota, it is the default.  You'd never specify it.  There are 10
> quota options at the bottom of the XFS section of mount(8) that one
> might want to set.
>
> It is quite irritating, yet surprisingly common, to see XFS users
> re-specifying the defaults in their /etc/fstab, because they didn't take
> the time to educate themselves properly, and simply copy/pasted from one
> of many online "XFS tuning guides".  On the XFS list we call these "XFS
> mis-tuning guides", as nearly all of them contain mostly misinformation.
>   Not intentional mind you, but because they just don't know what they're
> talking about, or they did but the guide is 5+ years old, and things
> have changed.

Ok, so here's my final fstab... thanks again for all of the comments 
(especially yours Stan).

# <fs> <mountpoint>   <type>    <opts> <dump/pass>
#
# Non-LVM volumes
#
/dev/sda1       /boot         ext4     defaults,noauto      1 2
/dev/sda3       /             ext4     defaults             0 1
/dev/sda2       none          swap     sw                   0 0
/dev/cdrom      /mnt/cdrom    auto     noauto,ro            0 0
/dev/fd0        /mnt/floppy   auto     noauto               0 0
#
# LVM volumes
#
/dev/vg/var     /var          xfs      defaults,inode64     0 2
/dev/vg/tmp     /tmp          ext2     nodev,noexec,nosuid  0 2
/dev/vg/vtmp    /var/tmp      ext2     nodev,nosuid         0 2
/dev/vg/log     /var/log      ext2     defaults             0 2
/dev/vg/snaps   /snaps        xfs      defaults,inode64     0 2

-- 

Best regards,

Charles