Other activity, e.g. syslog? Journaling at the FS level? you could try making a small partition formatted as ext2.
On Tue, Aug 10, 2010 at 5:10 PM, Jeremy Davis <jerdavis.cassan...@gmail.com> wrote: > Yeah, it has a BBU, and it is charged and on.. > Very odd behavior, I'm stumped. > > -JD > > On Tue, Aug 10, 2010 at 12:28 AM, Peter Schuller > <peter.schul...@infidyne.com> wrote: >> >> I have no explanation for the slower reads, but I have an hypothesis >> on the writes. >> >> Your iostat shows: >> >> > Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s >> > avgrq-sz avgqu-sz await svctm %util >> > cciss/c0d0 0.00 908.50 0.00 110.50 0.00 8152.00 >> > 73.77 0.57 5.20 4.93 54.50 >> > cciss/c0d1 0.00 0.00 16.50 0.00 1424.00 0.00 >> > 86.30 0.10 6.06 2.73 4.50 >> > dm-0 0.00 0.00 0.00 1019.00 0.00 8152.00 >> > 8.00 6.25 6.13 0.53 54.50 >> > dm-1 0.00 0.00 0.00 0.00 0.00 0.00 >> > 0.00 0.00 0.00 0.00 0.00 >> >> So that's keeping the disk busy around 50-60% of the time. This seems >> roughly consistent with your commit batch window being set to 5 ms and >> the system drive NOT being supported by a battery-backed cache (such >> that an fsync() actually does have to wait for the data to be on the >> platter). >> >> Is your non-system drive backed by the BBU? (I'm not sure if the >> controllers would support having some volumes backed by BBU protected >> cache and not others.) >> >> If it is the case that the other volume is BBU backed, then maybe the >> slowdown on writes is due to this. In any case, whatever the situation >> was before, the above stats do seem roughly consistent with >> write-through fsync() and batch window of 5ms, given sufficient >> concurrency to achieve the throughput you're seeing. On the other >> hand, "roughly consistent" is not very precise, and the original >> performance on the RAID:ed device is probably also roughly consistent >> with this ;) >> >> -- >> / Peter Schuller > > -- Jonathan Ellis Project Chair, Apache Cassandra co-founder of Riptano, the source for professional Cassandra support http://riptano.com
