[Dovecot] SSD drives are really fast running Dovecot

[Stan Hoeppner]

Ed W put forth on 1/16/2011 4:11 PM:
> 
>> Using XFS with delayed logging mount option (requires kernel 2.6.36 or later).
>>
>> XFS has natively used delayed allocation for quite some time, coalescing
>> multiple pending writes before pushing them into the buffer cache.  This not
>> only decreases physical IOPS, but it also decreases filesystem fragmentation by
>> packing more files into each extent.  Decreased fragmentation means fewer disk
>> seeks required per file read, which also decreases physical IOPS.  This also
>> greatly reduces the wasted space typical of small file storage.  Works very well
>> with maildir, but also with the other mail storage formats.
> 
> What happens if you pull out the wrong cable in the rack, kernel lockup/oops,
> power failure, hot swap disk pulled, or something else which causes an
> unexpected loss of a few seconds of written data?

Read the XFS FAQ.  These questions have been answered hundreds of times since
XFS was released in Irix in 1994.  I'm not your personal XFS tutor.

> Surely your IOPs are hard limited by the number of fsyncs (and size of any
> battery backed ram)?

Depends on how your applications are written and how often they call fsync.  Do
you mean BBWC?  WRT delayed logging BBWC is mostly irrelevant.  Keep in mind
that for delayed logging to have a lot of metadata writes in memory someone, or
many someones, must be doing something like an 'rm -rf' or equivalent on a large
dir with many thousands of files.  Even in this case, the processing is _very_ fast.

If your assumption is that your system is unstable, or you assume you will do
stupid things to break your system, then don't use a high performance
filesystem.  This behavior is not limited to XFS.

-- 
Stan