Having looked at this case, it seems like the issues are settled. +1
- Garrett
On Wed, 2010-07-28 at 07:17 +0800, bz211116 wrote:
> Nicolas Williams wrote:
> > On Wed, Jul 28, 2010 at 07:06:20AM +0800, bz211116 wrote:
> >
> >> Nicolas Williams wrote:
> >>
> >>> On Tue, Jul 27, 2010 at 12:47:31PM -0700, Garrett D'Amore wrote:
> >>>
> >>>> Is there a reason that this we would ever, under normal circumstances,
> >>>> want to use RMW on these devices? Is there a reason that this should
> >>>> even be exposed as a tunable to customers?
> >>>>
> >>> Probably when accessing UFS, FAT, and other such filesystems.
> >>>
> >> Yes, For other file systems or applications which still issue non-4K
> >> aligned I/Os to those SSDs which has low performance RMW in f/w, turn
> >> on RMW in sd can greatly improve the performance, our experiments show
> >> 100x faster than RMW in f/w. But for those disk drives which perform
> >> RMW better at f/w, we should turn RMW in sd driver off. This is one
> >> reason we need this tunable.
> >>
> >
> > But do we care about performance of non-ZFS here? (Answer: probably.
> > I'm thinking of raw device uses too.)
> >
> >
> >>> But I don't get why this needs to be a tunable either, at least for ZFS,
> >>> since we could ensure that ZFS always does 4KB aligned writes, and then
> >>> who cares if UFS, FAT, and friends run slow on flash.
> >>>
> >> For ZFS, which now uses READ CAPACITY 16 (if succeed) to get the
> >> physical block size of SSD. If the physical block size is 4096, ZFS
> >> aligns its minimal I/O size and request address at 4K boundary which
> >> can get best performance. But most of the SSDs now still report 512B
> >> physical sector size or even do not support physical sector size at
> >> all, and some of them also has bad RMW performance in f/w. in this
> >> case, we should turn on RMW in sd for them. that's another reason we
> >> need this tunable.
> >>
> >
> > But shouldn't ZFS just always do 4KB aligned writes and be done? Who
> > cares if the SSD claims to have a 512B physical sector size?
> >
> Most of SSDs in F5100/F20 is in this case...
>
> Thanks.
> -bo
> > Nico
> >
>
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