ZFS dedup write pathway - possible inefficiency or..?
avg at FreeBSD.org
Wed Apr 4 17:04:42 UTC 2018
On 02/04/2018 21:30, Stilez Stilezy wrote:
> Thanks on that tip, Andriy,
> I don't have the knowledge to tell if that's the first of the 2 issues I'm
> seeing. It could be. What exactly is that bug describing and what behaviour
> would it create?
Sub-optimal performance for ZFS write throughput (and latency) when dedup is
enabled and you are trying to write as fast as you can.
> Assuming it's the same -
> 1) Are there any known workarounds or sysctls that help to reduce the issue?
I don't know of any.
> 2) Are there any easy diagnostic tests I can easily do, to confirm if this is
> the same behaviour as that bug? Nobody else uses the test system, so I can
> change any sysctls to sane or extreme values for testing, or create large txg's
> and spread out reads and writes to see what's happening and what coincides with
We used DTrace to observe internal ZFS behavior.
I do not have any simple recipes.
> On 2 April 2018 at 18:47, Andriy Gapon <avg at freebsd.org
> <mailto:avg at freebsd.org>> wrote:
> On 02/04/2018 16:36, Stilez Stilezy wrote:
> > The first issue is specific to the
> > dedup write pathway. I've tested locally to a point where it seems it's
> > not due to inadequate hardware and it's very consistent and specific, even
> > on idle conditions/minimal load. I'm wondering whether there's a code
> > bottleneck specifically affecting just the dedup write pathway.
> I think that this might be https://www.illumos.org/issues/8353
> Andriy Gapon
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