Re: Revisiting disk layout on ZFS systems

On 4/28/2014 1:22 PM, Heikki Linnakangas wrote:
> On 04/28/2014 09:07 PM, Karl Denninger wrote:
>>> The WAL is fsync'd frequently. My guess is that that causes a lot of
>>> extra work to repeatedly recompress the same data, or something like
>>> that.
>>
>> It shouldn't as ZFS re-writes on change, and what's showing up is not
>> high I/O*count* but rather percentage-busy, which implies lots of head
>> movement (that is, lots of sub-allocation unit writes.)
>
> That sounds consistent frequent fsyncs.
>
>> Isn't WAL essentially sequential writes during normal operation?
>
> Yes, it's totally sequential. But it's fsync'd at every commit, which
> means a lot of small writes.
>
> - Heikki

Makes sense; I'll muse on whether there's a way to optimize this
further... I'm not running into performance problems at present but I'd
rather be ahead of it....

--
-- Karl
karl(at)denninger(dot)net