On Thu, Feb 25, 2021 at 8:51 PM Vladimir Sementsov-Ogievskiy < vsement...@virtuozzo.com> wrote:
> 19.02.2021 19:58, Eric Blake wrote: > > On 2/19/21 10:42 AM, Eric Blake wrote: > > > >>> To me, data=false looks compatible with NBD_STATE_HOLE. From user point > >>> of view, getting same results from qemu-nbd and qemu-img is more > >>> important than being more correct about allocation status. > >> > >> More to the point, here is our inconsistency: > >> > >> In nbd/server.c, we turn !BDRV_BLOCK_ALLOCATED into NBD_STATE_HOLE > >> > >> In block/nbd.c, we turn !NBD_STATE_HOLE into BDRV_BLOCK_DATA > >> > >> The fact that we are not doing a round-trip conversion means that one of > >> the two places is wrong. And your argument that the server side is > >> wrong makes sense to me. > > > > In fact, when I went back and researched when this was introduced (see > > commit e7b1948d51 in 2018), we may have been aware of the inconsistency > > between client and server, but didn't make up our minds at the time: > > https://lists.gnu.org/archive/html/qemu-devel/2018-03/msg03465.html > > "? Hm, don't remember, what we decided about DATA/HOLE flags mapping.." > > > >> > >> I'll wait a few days for any other reviewer commentary before taking > >> this through my NBD tree. > >> > > > > > I can add the following. > > First, link to my research of block_status in Qemu: > https://lists.gnu.org/archive/html/qemu-devel/2020-04/msg05136.html > > And about HOLE and ZERO.. > > As I've noted in the research above, SCSI may return HOLE & !ZERO: > > from SCSI: > Logical Block Provisioning Read Zeros (LBPRZ) bit > 1 If the logical block provisioning read zeros (LBPRZ) bit is set to > one, then, for an unmapped LBA specified by a read operation, the > deviceserver shall send user data with all bits set to zero to the data-in > buffer. > 0 If the TPRZ bit is set to zero, then, for an unmapped LBA specified > by a read operation, the device server may send user data with all bitsset > to any value to the data-in buffer. > > So we can have an unmapped area that can be read as any random data. Same > thing can be said about null-co driver with read-zeroes=false > > Also, qcow2 support ALLOCATED ZERO clusters which reads as zero but data > is allocated - they are reasonable to report as ZERO & !HOLE > > And of-course UNALLOCATED ZERO clusters in qcow2 and lseek-holes are > reasonable to report as ZERO & HOLE, because they reads as zero and > "future writes to that area may cause fragmentation or encounter an > NBD_ENOSPC".. > > So, all combination are reasonable, we just need to fix Qemu NBD server to > report correct statuses in all these cases. > > It seems that ZERO/HOLE specification is a lot more reasonable than what > we have with ZERO/DATA/ALLOCATED in Qemu, and may be true way is move > internal block_status to use NBD terms. > > > And thanks for CCing me. Hmm, maybe, I'll suggest myself as co-maintainer > for NBD? Kevin, Max, are you ok with this change?
