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SubjectRe: [PATCH 0/7] discard support revisited
On Sat, Aug 29, 2009 at 10:15:34PM -0400, Christoph Hellwig wrote:
> On Sat, Aug 29, 2009 at 05:37:19PM -0600, Matthew Wilcox wrote:
> > I think we're going to need to figure out whether we should be sending
> > UNMAP or WRITE SAME ... probably need to dive back into the T10 poostorm
> > to see what's going on.
> Good question. Latest I had heard was that at least one array vendor
> prefers the WRITE SAME. To me it looks like the much saner interface
> for the OS, so unless there are arrays that strongly prefer UNMAP or
> we need to make use of the multiple extends feature in it I'd go with
> WRITE SAME as first choice.

I think we're going to see a split in array vendors, tbh. Many were
very upset at the thought of taking out multiple extents from the UNMAP
command. Which I suggested, because frankly it's insane.

> > Jens had some objections to the block layer bits last time I posted
> > these. I forget what they were now (this would have been around May
> > 2nd, I think). What I've done instead in my current patchset (which
> > undoubtedly has bugs because it isn't tested, because I'm not supposed
> > to be working on the weekends) is to make sd_prep_fn() call a new method
> > in the scsi_host_template. That should translate the discard request
> > into a BLOCK_PC ATA_16 command, and we'll all be happy.
> >
> > It goes a little something like this:
> >;a=shortlog;h=trim-20090829
> >
> > Right now, the test tool is telling me 'Operation not supported', and
> > I haven't tried to figure out why yet.
> Queue flag and handling the discard in the prep function is much better
> than the prepare function, yes. I don't like the prep_fn callout to the
> host a lot.

No, but I think we can make it more palatable. Look at the ugly USB
hack for accessing near the end of the disc that we have in sd_prep_fn
right now. If we can push that into the USB driver, I think that'll make
everybody happier.

This also gives us an interesting opportunity to experiment with
translating read/write commands directly into ATA_16 commands rather than
going through the SCSI translation first. That should save a few cycles.

> If we go with WRITE SAME as prefered discard option for
> scsi translating it to TRIM should be relatively easy, it uses the same
> LBA/length encoding as the regular WRITE_16, except that the payload is
> just a single sector. That should be not too hard to implement in the
> SAT layer.

It should avoid the difficulty in translating the command size, true.

Matthew Wilcox Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours. We can't possibly take such
a retrograde step."

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