Messages in this thread | | | From | David Rees <> | Date | Wed, 10 Mar 2010 15:44:54 -0800 | Subject | Re: howto combat highly pathologic latencies on a server? |
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On Wed, Mar 10, 2010 at 9:17 AM, Hans-Peter Jansen <hpj@urpla.net> wrote: > While this system usually operates fine, it suffers from delays, that are > displayed in latencytop as: "Writing page to disk: 8425,5 ms": > ftp://urpla.net/lat-8.4sec.png, but we see them also in the 1.7-4.8 sec > range: ftp://urpla.net/lat-1.7sec.png, ftp://urpla.net/lat-2.9sec.png, > ftp://urpla.net/lat-4.6sec.png and ftp://urpla.net/lat-4.8sec.png. > > From other observations, this issue "feels" like it is induced by single > syncronisation points in the block layer, eg. if I create heavy IO load on > one RAID array, say resizing a VMware disk image, it can take up to a > minute to log in by ssh, although the ssh login does not touch this area at > all (different RAID arrays). Note, that the latencytop snapshots above are > made during normal operation, not this kind of load.. > > Might later kernels mitigate this problem? As this is a production system, > that is used 6.5 days a week, I cannot do dangerous experiments, also > switching to 64 bit is a problem due to the legacy stuff described above... > OTOH, my users suffer from this, and anything helping in this respect is > highly appreciated.
Seems like a 2.6.32 based kernel which has per-BDI writeback and "CFQ low latency mode" changes might help a good deal. I know that on one of my bigger machines (similar in specs to yours) which has a lot of processes which do a decent amount of IO, latency and load average has gone down after going to a 2.6.32 kernel from a 2.6.31 kernel (Fedora 11 system).
Like Chris suggested, I've also heard that using the noop IO scheduler can work well on Areca controllers on some kernels and workloads. It's worth a shot and you can even try changing it at run-time.
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