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

[Kip Macy]

On Wed, Jun 10, 2009 at 12:00 AM, Dmitry Morozovsky<marck at rinet.ru> wrote:
> On Tue, 9 Jun 2009, Kip Macy wrote:
>
> KM> > KM>   As far as I can tell systems that have less than 4GB are more often hurt
> KM> > KM>   by prefetched than helped.  On i386 systems and systems with less than 4GB,
> KM> > KM>   prefetch is now disabled by default. I've added a prefetch enable tunable, to
> KM> > KM>   enable prefetching for those systems. The prefetch disable tunable will continue
> KM> > KM>   to unconditionally disable prefetching.
> KM> > KM>
> KM> >
> KM> > [snip]
> KM> >
> KM> > KM> +extern int zfs_prefetch_disable;
> KM> > KM> +extern int zfs_prefetch_enable;
> KM> >
> KM> > [snip]
> KM> >
> KM> > Before the interface is not settled in stone, don't you think simple bool
> KM> > zfs_prefetch tunable (defauled to what looks more reasonable to partucilar
> KM> > arch/memsize) is more clean and straightforward?
> KM> >
> KM>
> KM> Yes. But the semantics I want don't lend themselves well to a boolean.
> KM> I want it to default to off for i386 and amd64 w/ < 4GB and on for
> KM> amd64 > 4GB. However, I want it to be possible for users with larger
> KM> memory systems to disable it and users with smaller memory systems to
> KM> enable it. The only way a simple boolean would work would be if it
> KM> were zfs_prefetch_non_default_behavior, which I think is even more
> KM> confusing than what I have now.
>
> Hmm, I failed to see how boolean prevent the usage:
>
> if arch==i386 || memsize < 4G
>        zfs_prefetch=0
> else
>        zfs_prefetch=1
>
> and admin can flip the switch to non-default position.
>

Right, what you're advocating is zfs_prefetch_non_default. Which is
simpler, but still rather weird.

I'll ask around.

Cheers,
Kip