Re: [RFC PATCH v2 25/26] qcow2: Allow preallocation and backing files if extended_l2 is set

[Max Reitz <mreitz@redhat.com>]

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On 26.10.19 23:25, Alberto Garcia wrote:
> Traditional qcow2 images don't allow preallocation if a backing file
> is set. This is because once a cluster is allocated there is no way to
> tell that its data should be read from the backing file.
> 
> Extended L2 entries have individual allocation bits for each
> subcluster, and therefore it is perfectly possible to have an
> allocated cluster with all its subclusters unallocated.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
>  block/qcow2.c              | 7 ++++---
>  tests/qemu-iotests/206.out | 2 +-
>  2 files changed, 5 insertions(+), 4 deletions(-)

But it doesn’t work, because qcow2_alloc_cluster_offset() always
allocates the whole cluster, so the backing file content isn’t visible:

$ ./qemu-img create -f qcow2 base.qcow2 64M
Formatting 'base.qcow2', fmt=qcow2 size=67108864 cluster_size=65536
lazy_refcounts=off extended_l2=off refcount_bits=16

$ ./qemu-io -c 'write -P 42 0 64M' base.qcow2
wrote 67108864/67108864 bytes at offset 0
64 MiB, 1 ops; 00.21 sec (307.344 MiB/sec and 4.8022 ops/sec)

$ ./qemu-img create -f qcow2 -o preallocation=metadata,extended_l2=on \
    top.qcow2
Formatting 'top.qcow2', fmt=qcow2 size=67108864 backing_file=base.qcow2
cluster_size=65536 preallocation=metadata lazy_refcounts=off
extended_l2=on refcount_bits=16

$ ./qemu-io -c 'read -P 42 0 64M' top.qcow2
Pattern verification failed at offset 0, 67108864 bytes
read 67108864/67108864 bytes at offset 0
64 MiB, 1 ops; 00.03 sec (2.498 GiB/sec and 39.9725 ops/sec)

Max


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