[CentOS] 3Ware 9550SX and latency/system responsiveness

Ross S. W. Walker rwalker at medallion.com
Wed Oct 3 14:40:40 UTC 2007

Simon Banton wrote:
> At 12:59 -0400 2/10/07, Ross S. W. Walker wrote:
> >Try running the same benchmark but use bs=4k and count=1048576
> Just finished doing that now - comparison graphs are here:
> http://community.novacaster.com/showarticle.pl?id=7492
> >While these tests are running can you run any processes on another
> >session?
> Yes, but responsiveness is sluggish eg (taken during 4k 'deadline' 
> scheduler test)
> # time ls /usr/lib
> real    0m15.959s
> user    0m0.011s
> sys     0m0.016s

Well it looks like the CFQ actually was able to get some reads in
while the background write was going on so it actually looks
better in this workload scenario.

I am going to retract my suggestion that 'deadline' be a general
purpose scheduler for servers based on this. Instead I would make
the following alternate suggestions.

The issue I had with CFQ is it cannot handle overlapping IO well
from multiple threads of the same process, so if your application
does that (MySQL?) then it is probably NOT the right scheduler for
you and you might be best to consider 'deadline' or 'noop' and
put your data on a separate disk/array that doesn't compete with
any other process/application.

If an application spawns multiple threads or processes to handle
completely separate data workloads (ie not overlapping io from
same workload) then you are best using the default 'cfq' I believe.

I would try some web benchmark app next with both cfq and then
deadline to see which works better in a web-server environment. For
web serving that is read only I suspect that either 'cfq' or
'deadline' will work well, but would like to know the results of
your web benchmarks.

In the end, since all your "content" will be in mysql and therefore
all file system operations will be "read", the whole issue of being
able to read while writing a large file isn't very relevant, so I
would probably disregard it as an edge case that doesn't fit your


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