Check here:

 
http://cvs.opensolaris.org/source/xref/on/usr/src/uts/common/fs/zfs/vdev_disk.c#157

-r

Phil Brown writes:
 > Roch Bourbonnais - Performance Engineering wrote:
 > > I'm puzzled by 2 things.
 > > 
 > > Naively I'd think a write_cache  should not help throughput
 > > test since the cache should fill  up after which you should still be
 > > throttled by the physical drain rate. You clearly show that
 > > it helps; Anyone knows why/how a cache helps throughput ?
 > > 
 > > And the second thing...quick search, this seems relevant
 > > 
 > >    Bug ID: 6397876
 > >    Synopsis: sata drives need default write cache controlled via property
 > >    Integrated in Build: snv_38
 > >    http://bugs.opensolaris.org/bugdatabase/view_bug.do?bug_id=6397876
 > > 
 > 
 > 
 > well, that just says that the cache settings dont stick across reboot. that 
 > doesnt seem to have any bearing on whether zfs toggles write cache or not.
 > 
 >  From the sound of things, it sounds like what was previously written on 
 > this list was incorrect: I now infer that zfs does NOT do any "smart 
 > toggling" of write cache enable/disable on drives that it uses.
 > (although it may or may not do some "flush cache" calls at appropriate 
 > moments)
 > 
 > 
 > _______________________________________________
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 > zfs-discuss@opensolaris.org
 > http://mail.opensolaris.org/mailman/listinfo/zfs-discuss

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