On 2/25/21 8:57 AM, Vladimir Sementsov-Ogievskiy wrote: > 25.02.2021 16:50, Vladimir Sementsov-Ogievskiy wrote: >> 18.02.2021 23:15, Eric Blake wrote: >>> Previous commits (such as 6e280648, 75d34eb9) have mentioned that our >>> NBD server still sends unaligned fragments when an active layer with >>> large advertised minimum block size is backed by another layer with a >>> smaller block size. Expand the test to actually cover these scenario, >>> by using two different approaches: qcow2 encryption (which forces >>> 512-byte alignment) with an unaligned raw backing file, and blkdebug >>> with a 4k alignment. >>>
> > Now I don't think that aligning qemu:allocation-depth information is a > correct thing to do. Why not? First, it's very rare that you'd have a qcow2 image with mandated 4k minimum block size, backed by another qcow2 image with 512 block size (blkdebug made it possible to expose the bug, but I could not find a way in common day-to-day usage), so we really aren't impacting normal users. Second, from the perspective of copying backing chains over NBD, what difference does it make if we have the backing chain: A (granularity 512) <- B (granularity 512) <- C (granularity 4k) with the allocation pattern: A: -A-A-A-A-A-A-A-A B: --BB--BB--BB--BB C: --------CCCCCCCC and report the allocation depth as: 2222222211111111 instead of 0322032211111111 The former may be imprecise, but it obeys our bounds, and in all reality, if all we have access to is 4k chunks, any decisions we make about how to handle that 4k block should be based on the fact that at least some of the data was allocated in our backing file, and not treating the entire 4k as unallocated merely because the first 512 bytes are neither in A nor B. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org
