Hi all! I wanted to understand, what is the real difference between bdrv_block_status_above and bdrv_is_allocated_above, IMHO bdrv_is_allocated_above should work through bdrv_block_status_above..
And I found the problem: bdrv_is_allocated_above considers space after EOF as UNALLOCATED for intermediate nodes.. UNALLOCATED is not about allocation at fs level, but about should we go to backing or not.. And it seems incorrect for me, as in case of short backing file, we'll read zeroes after EOF, instead of going further by backing chain. This leads to the following effect: ./qemu-img create -f qcow2 base.qcow2 2M ./qemu-io -c "write -P 0x1 0 2M" base.qcow2 ./qemu-img create -f qcow2 -b base.qcow2 mid.qcow2 1M ./qemu-img create -f qcow2 -b mid.qcow2 top.qcow2 2M Region 1M..2M is shadowed by short middle image, so guest sees zeroes: ./qemu-io -c "read -P 0 1M 1M" top.qcow2 read 1048576/1048576 bytes at offset 1048576 1 MiB, 1 ops; 00.00 sec (22.795 GiB/sec and 23341.5807 ops/sec) But after commit guest visible state is changed, which seems wrong for me: ./qemu-img commit top.qcow2 -b mid.qcow2 ./qemu-io -c "read -P 0 1M 1M" mid.qcow2 Pattern verification failed at offset 1048576, 1048576 bytes read 1048576/1048576 bytes at offset 1048576 1 MiB, 1 ops; 00.00 sec (4.981 GiB/sec and 5100.4794 ops/sec) ./qemu-io -c "read -P 1 1M 1M" mid.qcow2 read 1048576/1048576 bytes at offset 1048576 1 MiB, 1 ops; 00.00 sec (3.365 GiB/sec and 3446.1606 ops/sec) I don't know, is it a real bug, as I don't know, do we support backing file larger than its parent. Still, I'm not sure that this behavior of bdrv_is_allocated_above don't lead to other problems. ===== Hmm, bdrv_block_allocated_above behaves strange too: with want_zero=true, it may report unallocated zeroes because of short backing files, which are actually "allocated" in POV of backing chains. But I see this may influence only qemu-img compare, and I don't see can it trigger some bug.. with want_zero=false, it may do no progress because of short backing file. Moreover it may report EOF in the middle!! But want_zero=false used only in bdrv_is_allocated, which considers onlyt top layer, so it seems OK. ===== So, I propose these series, still I'm not sure is there a real bug. Vladimir Sementsov-Ogievskiy (4): block/io: fix bdrv_co_block_status_above block/io: bdrv_common_block_status_above: support include_base block/io: bdrv_common_block_status_above: support bs == base block/io: fix bdrv_is_allocated_above block/io.c | 104 ++++++++++++++++++------------------- tests/qemu-iotests/154.out | 4 +- 2 files changed, 53 insertions(+), 55 deletions(-) -- 2.21.0
