svn commit: r193878 - head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs

Kip Macy kmacy at
Wed Jun 10 06:24:52 UTC 2009

On Tue, Jun 9, 2009 at 11:15 PM, Dmitry Morozovsky<marck at> wrote:
> Kip,
> On Wed, 10 Jun 2009, Kip Macy wrote:
> KM> Author: kmacy
> KM> Date: Wed Jun 10 01:21:32 2009
> KM> New Revision: 193878
> KM> URL:
> KM>
> KM> Log:
> KM>   As far as I can tell systems that have less than 4GB are more often hurt
> KM>   by prefetched than helped.  On i386 systems and systems with less than 4GB,
> KM>   prefetch is now disabled by default. I've added a prefetch enable tunable, to
> KM>   enable prefetching for those systems. The prefetch disable tunable will continue
> KM>   to unconditionally disable prefetching.
> KM>
> [snip]
> KM> +extern int zfs_prefetch_disable;
> KM> +extern int zfs_prefetch_enable;
> [snip]
> Before the interface is not settled in stone, don't you think simple bool
> zfs_prefetch tunable (defauled to what looks more reasonable to partucilar
> arch/memsize) is more clean and straightforward?

Yes. But the semantics I want don't lend themselves well to a boolean.
I want it to default to off for i386 and amd64 w/ < 4GB and on for
amd64 > 4GB. However, I want it to be possible for users with larger
memory systems to disable it and users with smaller memory systems to
enable it. The only way a simple boolean would work would be if it
were zfs_prefetch_non_default_behavior, which I think is even more
confusing than what I have now.


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