[Dovecot] XFS vs EXT4 for mail storage

[Stan Hoeppner]

On 5/3/2013 6:30 AM, Charles Marcus wrote:
> 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).

When I said "lots of small random IOs" I was leading into the
explanation of why alignment isn't necessary, and actually detrimental
to a mail workload.  It's WRT filesystem alignment to the RAID stripe
that maildir and mdbox are little different.

> 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.

Digesting the inner workings of a filesystem, especially one as complex
and tweakable as XFS, and how they relate to real world workloads, is
not for the faint of heart.  Ironically, today's XFS defaults work
extremely well "out of the box" for many workloads, including mail.

>> 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... :)

I need to get this Dovecot doc thing finished first...

> 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

3.7, yeah, pretty sure delaylog is no longer an option with this recent
a kernel.
...
> Ok, so here's my final fstab... thanks again for all of the comments
> (especially yours Stan).
> 
> # <fs> <mountpoint>   <type>    <opts> <dump/pass>

> /dev/vg/var     /var          xfs      defaults,inode64     0 2
...
> /dev/vg/snaps   /snaps        xfs      defaults,inode64     0 2

I assume /var will hold user mail dirs.  Do /var/ and /snaps reside on
the same RAID array, physical disks?  How about the other filesystems I
snipped?  If you have a large number of filesystems atop the same RAID,
some of them being XFS, this could create a head thrashing problem under
high load increasing latency and thus response times.

Would you mind posting:

~$ xfs_info /dev/vg/var
~$ xfs_info /dev/vg/snaps

-- 
Stan