Disable zfs prefetch by default

[Fabian Keil]

"Ilya A. Arkhipov" <rum1cro at yandex.ru> wrote:

> We have zfs prefetch enabled by default with > 4GB, but I didn't seen 
> any case for using it, and all tuning guide saying to disable it.

Which tuning guides are you referring to?

> Maybe we disable zfs prefetch by default ? Or could you please provide 
> why it needed.

Whether or not prefetching is useful depends on the workload
and the setup.

If you doubt that the prefetch default is useful for your workload
(for example because your systems mainly do small "random" reads
from "random" files), you can easily test this by disabling prefetching
for a while.

If this is too much work (or if no difference can be measured),
the default is probably fine.

Fabian
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