Hi Kevin, Thanks for the explanation!
Lin ________________________________ From: Kevin Wolf <kw...@redhat.com> Sent: Tuesday, June 2, 2020 7:06 PM To: Lin Ma <l...@suse.com> Cc: qemu-devel@nongnu.org <qemu-devel@nongnu.org>; pbonz...@redhat.com <pbonz...@redhat.com> Subject: Re: [question] Partial sector issue while discard in qcow2 image Am 02.06.2020 um 09:45 hat Lin Ma geschrieben: > Hi all, > > During woring to add GET LBA STATUS support in qemu scsi emulation > layer, I encountered an unmap issue with qcow2 image, It's likely > about how to unmap partial clusters. e.g.: > > With these default values: > * the default value of s->qdev.blocksize: 512 > * the default value of s->cluster_size of qcow2 image: 65536 > > Running 'sg_unmap -l 1024 -n 32 /dev/sda' hits the condition > 'if (!QEMU_IS_ALIGNED(offset | bytes, s->cluster_size))' in the > function qcow2_co_pdiscard, Then it won't perform > qcow2_cluster_discard(), So the corresponding clusters won't be > discard in this situation, Correct? Yes, this is correct. Discard is just a hint, so doing nothing is a perfectly valid implementation. In the case of qcow2, only full clusters are discarded. > Of cause, with the default blocksize and cluster_size, The below > examples won't hit the condition 'if (!QEMU_IS_ALIGNED...'. > sg_unmap -l 1024 -n 128 /dev/sda > or > sg_unmap -l 256 -n 128 /dev/sda Yes, and when discarding whole block devices (e.g. while creating a new filesystem on them) or large files, you'll probably get this case for most parts. Kevin