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 _______________________________________________ OpenIndiana-discuss mailing list [email protected] http://openindiana.org/mailman/listinfo/openindiana-discuss
