Re: Revisiting disk layout on ZFS systems

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