Re: Revisiting disk layout on ZFS systems

On 4/28/2014 1:04 PM, Heikki Linnakangas wrote:
> On 04/28/2014 06:47 PM, Karl Denninger wrote:
>> What I am curious about, however, is the xlog -- that appears to suffer
>> pretty badly from 128k record size, although it compresses even
>> more-materially; 1.94x (!)
>>
>> The files in the xlog directory are large (16MB each) and thus "first
>> blush" would be that having a larger record size for that storage area
>> would help. It appears that instead it hurts.
>
> 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.
>
> - Heikki
>
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.)

Isn't WAL essentially sequential writes during normal operation?

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