[Gluster-users] temp fix: Simultaneous reads and writes from specific apps to IPoIB volume seem to conflict and kill performance.

[Harry Mangalam]

The problem described in the subject appears NOT to be the case.  It's
not that simultaneous reads and writes dramatically decrease perf, but
that the type of /writes/ being done by this app (bedtools) kills
performance. If this was a self-writ app or an infrequently used one,
I wouldn't bother writing this up, but bedtools is a fairly popular
genomics app and since many installations use gluster to host Next-Gen
sequencing data and analysis, I thought I'd follow up on my own post.

The short version:
=============
 Insert gzip to compress and stream the data before sending it to
gluster fs.  The improvement in IO (and application) performance is
dramatic.

ie (all files on a gluster fs)

genomeCoverageBed -ibam RS_11261.bam  -g \
ref/dmel-all-chromosome-r5.1.fasta -d |gzip > output.cov.gz

inserting the '| gzip' increased the app speed by more than 30X
(relative to not using it on a gluster fs; however it even improved
the wall clock speed of the app relative to running on a local
filesystem by about 1/3), decreased the gluster CPU utilization by
~99% and reduced the output size by 80%. So, wins all round.


The long version:
============
  The type of writes that bedtools does is also fairly common - lots
of writes of tiny amounts of data.

As I understand it (which may be wrong; please correct) the gluster
native client (which we're using) does not buffer IO as well as the
NFS client, which is why we frequently see complaints about gluster vs
NFS perf.
The apparent problem for bedtools is that these zillions of tiny
writes are being handled separately or at least not cached well to be
consolidated in a large write.  To present the data to gluster as a
continuous stream instead of these tiny writes, they have to be
'converted' to such a stream. gzip is a nice solution because it
compresses as it converts.  Aparently anything that takes STDIN,
buffers it appropriately and then spits it out on STDOUT will work.
Even piping the data thru 'cat' will work to allow bedtools to
continue to run at 100%, tho it will increase the gluster CPU
utilization to >90%. 'cat' of course uses less CPU (~14%) while gzip
will use more  (~60%) tho decreasing gluster;s use enormously.

I did try the performance options I mentioned earlier:

performance.write-behind-window-size: 1024MB
performance.flush-behind: on

  They did not seem to help at all and I'd still like an explanation
of what they're supposed to do.

The upshot is that this seems like, if not a bug, then at least an
opportunity to improve gluster perfomance considerably.

-- 
Harry Mangalam - Research Computing, OIT, Rm 225 MSTB, UC Irvine
[m/c 2225] / 92697 Google Voice Multiplexer: (949) 478-4487
415 South Circle View Dr, Irvine, CA, 92697 [shipping]
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