Il 17/07/2013 12:27, Kevin Wolf ha scritto: > Am 17.07.2013 um 12:21 hat Peter Lieven geschrieben: >> >> Am 17.07.2013 um 11:58 schrieb Paolo Bonzini <pbonz...@redhat.com>: >> >>> Il 17/07/2013 10:46, Kevin Wolf ha scritto: >>>> Am 15.07.2013 um 12:49 hat Peter Lieven geschrieben: >>>>> if a destination has has_zero_init = 0, but it supports >>>>> discard zeroes use discard to convert the target >>>>> into an all zero device. >>>>> >>>>> Signed-off-by: Peter Lieven <p...@kamp.de> >>>> >>>> Wouldn't it be better to use bdrv_write_zeroes() and extend the >>>> implementation of that to use discard internally in those block drivers >>>> where it makes sense? >>>> >>>> Because here you're not really discarding (i.e. don't care about the >>>> sectors any more), but you want them to be zeroed. >>> >>> I thought the same yesterday when reviewing the series, but I'm not >>> convinced. >>> >>> Discarding is not always the right way to write zeroes, because it can >>> disrupt performance. It may be fine when you are already going to write >>> a sparse image (as is the case for qemu-img convert), but not in >>> general. So if you just used write_zeroes, it would have to fall under >>> yet another -drive option (or an extension to "-drive discard"). I >>> think what Peter did is a good compromise in the end. >>> >>> BTW, Peter and Ronnie: we were assuming that UNMAP with LBPRZ=1 always >>> zeroes blocks, but is that true for unaligned operations? >> >> Good question, I will pass it to ronnie. My guess is that the command will >> fail with >> a check condition if it failed to unmap the data. From what Ronnie sent >> earlier >> it should be guaranteed that the blocks are at least zero after the unmap >> command. >> >> As for the qemu-img patch this shouldn't matter. It uses always blocks of >> bdi->max_unmap >> which should be a multiple of the alignment. It also checks if sectors are >> deallocated >> after the unmap afterwards. If the unmap fails it falls back to >> has_zero_init =1. > > Well, you use bdrv_discard(), and ignoring discards is valid. Just > another reason to use bdrv_write_zeroes() instead.
He's only using it if discard_zeroes is true in the new BlockDriverInfo. We can define the semantics of that bit, and I think defining it as "ignored discards will still write zeroes" is a good thing (same as what SCSI targets do if you use WRITE SAME to do the discard operation). Paolo
