> -----Original Message----- > From: linux-scsi-ow...@vger.kernel.org [mailto:linux-scsi- > ow...@vger.kernel.org] On Behalf Of James Bottomley > Sent: Wednesday, 16 July, 2014 1:02 PM > To: martin.peter...@oracle.com > Cc: linux-kernel@vger.kernel.org; h...@infradead.org; > de...@linuxdriverproject.org; a...@canonical.com; k...@microsoft.com; > sta...@vger.kernel.org; linux-s...@vger.kernel.org; oher...@suse.com; > jasow...@redhat.com > Subject: Re: [PATCH 4/8] Drivers: scsi: storvsc: Filter WRITE_SAME_16 > > On Wed, 2014-07-16 at 13:47 -0400, Martin K. Petersen wrote: > > >>>>> "Christoph" == hch@infradead org <h...@infradead.org> writes: > > > > Christoph> Oh, we actually have devices that support WRITE SAME with > > Christoph> unmap, but not without? That's defintively a little strange. > > > > Yep :( > > > > There were several SSDs that did not want to support wearing out flash > > by writing gobs of zeroes and only support the UNMAP case. > > > > Christoph> Yes, and it did this intentionally. I really wouldn't expect > > Christoph> devices to support WRITE SAME with UNMAP but blow up on a > > Christoph> WRITE SAME without it (and not just simple fail it in an > > Christoph> orderly way). > > > > *sigh* > > > > Christoph> It definitively seems odd to default to trying WRITE SAME for > > Christoph> unmap for a device that explicitly tells us that it doesn't > > Christoph> support WRITE SAME. > > > > Maybe it's just a naming thing. I was really trying to convey > > no_req_write_same support, not no_write_same_10_or_16. > > > > Christoph> Note that I'm not against your patch - I suspect forcing us > > Christoph> to read EVPD pages even for devices that claim to be SPC-2 > > Christoph> will come in useful in various scenarios. > > > > I don't have a problem with a BLIST_PREFER_UNMAP flag or something like > > that. But BLIST_TRY_VPD_PAGES seems more generally useful and it does > > fix the problem at hand. That's why I went that route. > > Hang on ... unless we apply Christoph or my fix, we'll get the same > issue with every raid driver (that's about 10 I think) that set > no_write_same when they hit a >2TB RAID volume, so I think we need both > fixes. > > James >
WRITE SAME with the UNMAP bit set to one (and a few other conditions) guarantees that the data is zeroed out, while the UNMAP command is just a hint. They're not fully interchangeable. Which semantics are implied by REQ_DISCARD and these functions? One benefit of UNMAP is that UNMAP supports a list of discontiguous LBA ranges, whereas WRITE SAME just supports one LBA range. sd_setup_discard_cmnd is not taking advantage of this feature, though. Ideally, the block layer would merge multiple discards into one UNMAP command if they're stuck on the request queue for a while, like it merges adjacent reads and writes. That would pave the way for building up WRITE SCATTERED and READ GATHERED commands. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
