[Dovecot] SSD drives are really fast running Dovecot

[Stan Hoeppner]

Rick Romero put forth on 1/15/2011 10:34 AM:

> Also, if your filesystem is using 4k clusters, aren't you only using 1
> random IOPS for a 4k email?   It just sounds to me like if you plan
> 'smarter', anyone can avoid the excessive costs of SSD and get 'end user
> similar' performance with commodity hardware.

This depends heavily on which filesystem you use and the flow of mails/sec.  For
instance, if multiple writes of small (<4KB) files (maildir), or multiple writes
to a single file (mbox) on an XFS filesystem are pending simultaneously, because
XFS uses a delayed allocation scheme, a variable extent size, and because it is
optimized for parallel workloads, it can pack many small files into a single 4KB
extent (if they add up to 4KB or less) and write them to disk in a single IOP.
Likewise, with mbox storage XFS can coalesce multiple writes to the same file
extent in a single IOP.  Moreover, XFS can take a pending write of say, 37 small
files or write fragments to the same file, and if they fit within say, 12KB, it
can write all 37 in 3 pipelined IOPS.  Using O_DIRECT with mbox files, the IOPS
performance can be even greater.  However, I don't know if this applies to
Dovecot because AFAIK MMAP doesn't work well with O_DIRECT...

***Hay Timo, does/can Dovecot use Linux O_DIRECT for writing the mail files?

Now, here's where load issue comes in.  If the pending writes are more than a
few seconds apart, you lose this XFS coalesced write advantage.  I don't know
how other filesystems handle writing multiple small files <4KB.

-- 
Stan