ZFS "stalls" -- and maybe we should be talking about defaults?

Steven Hartland killing at multiplay.co.uk
Thu Mar 7 20:45:47 UTC 2013

----- Original Message ----- 
From: "Karl Denninger" <karl at denninger.net>

>>> I will test that but first I have to get the test machine to reliably
>>> stall so I know I'm not chasing my tail.
>> Very sensible.
>> Assuming you can reproduce it, one thing that might be interesting to
>> try is to eliminate all sync IO. I'm not sure if there are options in
>> Postgres to do this via configuration or if it would require editing
>> the code but this could reduce the problem space.
>> If disabling sync IO eliminated the problem it would go a long way
>> to proving it isn't the IO volume or pattern per say but instead
>> related to the sync nature of said IO.
> That can be turned off in the Postgres configuration.  For obvious
> reasons it's a very bad idea but it is able to be disabled without
> actually changing the code itself.
> I don't know if it shuts off ALL sync requests, but the documentation
> says it does.
> It's interesting that you ran into this with RRD going; the machine in
> question does pull RRD data for Cacti, but it's such a small piece of
> the total load profile that I considered it immaterial.
> It might not be.

We never did get to the bottom of it but did come up with a fix.

Instead of using straight RRD interaction we switched all out code to
use rrdcached and put the files on SSD based pool, never had an issue


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