[Dovecot] XFS vs EXT4 for mail storage

[Stan Hoeppner]

On 5/2/2013 6:16 AM, Charles Marcus wrote:
...
> I've decided to use mdbox for storage (been using maildir), and will
> enable SIS for attachments.
> 
> So, anyone (Stan?) have any suggestions? Should I go with EXT4? Or XFS
> with just the defaults? Or XFS with one or more tuned parameters?
> 
> Appreciate any suggestions (including links to docs dealing with tuning
> XFS for my mail storage conditions that are written more at the layman
> level) or comments from anyone experienced using both...

>From a filesystem perspective mdbox is little different from maildir as
they both exhibit lots of small random IOs.  With either one aligning
the filesystem to the RAID stripe is problematic as it can create
spindle hotspots and increase free space fragmentation.  If you're using
a vmdk stripe alignment isn't possible anyway as VMware ignores hardware
device geometry WRT vmdks.

Although the EXT developers have been working overtime the last few
years trying to borrow/steal/duplicate the advanced performance features
of XFS, they have a very long way to go.  The parallel performance of
EXT is far behind as well as file allocation/layout and free space
management, to name a few.

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.

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.

-- 
Stan