On Mon, Nov 09, 2015 at 05:08:38PM -0600, Eric Sandeen wrote: > Hi Martin (and linux-ext4 list...) > > tl;dr: > > mke2fs today thinks that if discard_zeroes_data == 1 and a BLKDISCARD ioctl > of the whole device succeeds, then we have guarantees that any blocks read > back will be full of zero, and we don't need to initialize them to zero. Is > this ok? (barring crappy hardware, that is). > > slightly longer: > > Does discard_zeroes_data == 1 mean that a discard *request* will guarantee > zeroes on a read, or does it mean that a > discard-request-which-actually-was-executed-and-not-ignored-as-just-a-hint > will give us back zeroes on a read? (because UNMAP is a hint, right? I > don't know about SATA trim ...) > > I did see 7985090 sd: disable discard_zeroes_data for UNMAP - so I think that > for v3.19+, on *scsi*, what e2fsprogs is doing is ok (now). > > But I'm wondering about dm-thin and SATA, too, so trying to figure out what > discard_zeroes_data really implies. That after a BLKDISCARD, a read *will* > return zeros, or that it'll return zeros *iff* the hint is taken? Last winter I sent in a patch to invalidate the page cache after a discard: https://marc.info/?l=linux-kernel&m=142249686225748&w=2 ...because e2fsck gets confused it discards part of a d_z_d=1 device and gets non-zeroed buffers back (from the page cache!) immediately after. But it never went in. Should I resend it? Again? Jens never acted on it. --D > I hope that made sense, but I've been communicating badly today. ;) > > Thanks, > -Eric > -- > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
