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From Jeremy Davis <jerdavis.cassan...@gmail.com>
Subject Re: Using a separate commit log drive was 4x slower
Date Tue, 10 Aug 2010 21:10:49 GMT
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.schuller@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
>

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