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0da9856851
When querying image extents for raw image, qemu-nbd reports holes as
zero:
$ qemu-nbd -t -r -f raw empty-6g.raw
$ qemu-img map --output json nbd://localhost
[{ "start": 0, "length": 6442450944, "depth": 0, "zero": true, "data": true, "offset": 0}]
$ qemu-img map --output json empty-6g.raw
[{ "start": 0, "length": 6442450944, "depth": 0, "zero": true, "data": false, "offset": 0}]
Turns out that qemu-img map reports a hole based on BDRV_BLOCK_DATA, but
nbd server reports a hole based on BDRV_BLOCK_ALLOCATED.
The NBD protocol says:
NBD_STATE_HOLE (bit 0): if set, the block represents a hole (and
future writes to that area may cause fragmentation or encounter an
NBD_ENOSPC error); if clear, the block is allocated or the server
could not otherwise determine its status.
qemu-img manual says:
whether the sectors contain actual data or not (boolean field data;
if false, the sectors are either unallocated or stored as
optimized all-zero clusters);
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.
Changing nbd server to report holes using BDRV_BLOCK_DATA makes qemu-nbd
results compatible with qemu-img map:
$ qemu-img map --output json nbd://localhost
[{ "start": 0, "length": 6442450944, "depth": 0, "zero": true, "data": false, "offset": 0}]
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Message-Id: <20210219160752.1826830-1-nsoffer@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
29 lines
1.2 KiB
Plaintext
29 lines
1.2 KiB
Plaintext
QA output created by 241
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=== Exporting unaligned raw image, natural alignment ===
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size: 1024
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min block: 1
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[{ "start": 0, "length": 1000, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
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{ "start": 1000, "length": 24, "depth": 0, "zero": true, "data": false, "offset": OFFSET}]
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1 KiB (0x400) bytes allocated at offset 0 bytes (0x0)
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=== Exporting unaligned raw image, forced server sector alignment ===
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size: 1024
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min block: 512
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[{ "start": 0, "length": 1024, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
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1 KiB (0x400) bytes allocated at offset 0 bytes (0x0)
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WARNING: Image format was not specified for 'TEST_DIR/t.raw' and probing guessed raw.
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Automatically detecting the format is dangerous for raw images, write operations on block 0 will be restricted.
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Specify the 'raw' format explicitly to remove the restrictions.
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=== Exporting unaligned raw image, forced client sector alignment ===
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size: 1024
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min block: 1
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[{ "start": 0, "length": 1000, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
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{ "start": 1000, "length": 24, "depth": 0, "zero": true, "data": false, "offset": OFFSET}]
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1 KiB (0x400) bytes allocated at offset 0 bytes (0x0)
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*** done
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