[OpenIndiana-discuss] Usefulness of prefetch?

[Sašo Kiselkov]

On 07/09/2012 07:21 AM, Dan Swartzendruber wrote:
> Unless I am misunderstanding the above, we are almost never hitting on
> prefetched data, and barely ever on prefetched metadata.  Given that, is
> there even a reason to leave prefetch on?  I mean, it does generate extra
> reads, no?  

My experience with prefetch has been a nuanced one. I frequently use ZFS
for media streaming, which means I have dozens or hundreds of parallel
readers, each reading linearly. If I have lots of RAM, I leave prefetch
on, since it is quite effective at detecting these linear access
patterns (even though they're being issued by many applications,
essentially resulting in near-random request ordering). However, with
less RAM, I see prefetch over-stressing the I/O subsystem, since it
frequently prefetches too much data, fills up the prefetch buffers and
immediately dumps them (thus amplifying my read load significantly).

That being said, I would recommend you try another script, arcstat.pl -
that is a bit clearer and it gives you stats immediately, sort of in a
vmstat fashion, and post your results here.

Cheers,
--
Saso