qemu-img: Make unallocated part of backing chain obvious in map

The recently-added NBD context qemu:allocation-depth is able to
distinguish between locally-present data (even when that data is
sparse) [shown as depth 1 over NBD], and data that could not be found
anywhere in the backing chain [shown as depth 0]; and the libnbd
project was recently patched to give the human-readable name "absent"
to an allocation-depth of 0.  But qemu-img map --output=json predates
that addition, and has the unfortunate behavior that all portions of
the backing chain that resolve without finding a hit in any backing
layer report the same depth as the final backing layer.  This makes it
harder to reconstruct a qcow2 backing chain using just 'qemu-img map'
output, especially when using "backing":null to artificially limit a
backing chain, because it is impossible to distinguish between a
QCOW2_CLUSTER_UNALLOCATED (which defers to a [missing] backing file)
and a QCOW2_CLUSTER_ZERO_PLAIN cluster (which would override any
backing file), since both types of clusters otherwise show as
"data":false,"zero":true" (but note that we can distinguish a
QCOW2_CLUSTER_ZERO_ALLOCATED, which would also have an "offset":
listing).

The task of reconstructing a qcow2 chain was made harder in commit
0da9856851 (nbd: server: Report holes for raw images), because prior
to that point, it was possible to abuse NBD's block status command to
see which portions of a qcow2 file resulted in BDRV_BLOCK_ALLOCATED
(showing up as NBD_STATE_ZERO in isolation) vs. missing from the chain
(showing up as NBD_STATE_ZERO|NBD_STATE_HOLE); but now qemu reports
more accurate sparseness information over NBD.

An obvious solution is to make 'qemu-img map --output=json' add an
additional "present":false designation to any cluster lacking an
allocation anywhere in the chain, without any change to the "depth"
parameter to avoid breaking existing clients.  The iotests have
several examples where this distinction demonstrates the additional
accuracy.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210701190655.2131223-3-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
[eblake: fix more iotest fallout]
Signed-off-by: Eric Blake <eblake@redhat.com>

1 files changed, 24 insertions, 24 deletions

diff --git a/tests/qemu-iotests/274.out b/tests/qemu-iotests/274.out
index cfe17a8..16a95a4 100644
--- a/tests/qemu-iotests/274.out
+++ b/tests/qemu-iotests/274.out
@@ -26,18 +26,18 @@ read 1048576/1048576 bytes at offset 1048576
 0/1048576 bytes allocated at offset 1 MiB
 
 === Checking map ===
-[{ "start": 0, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": 327680}]
+[{ "start": 0, "length": 2097152, "depth": 0, "present": true, "zero": false, "data": true, "offset": 327680}]
 
 Offset          Length          Mapped to       File
 0               0x200000        0x50000         TEST_DIR/PID-base
 
-[{ "start": 0, "length": 1048576, "depth": 1, "zero": false, "data": true, "offset": 327680}]
+[{ "start": 0, "length": 1048576, "depth": 1, "present": true, "zero": false, "data": true, "offset": 327680}]
 
 Offset          Length          Mapped to       File
 0               0x100000        0x50000         TEST_DIR/PID-base
 
-[{ "start": 0, "length": 1048576, "depth": 2, "zero": false, "data": true, "offset": 327680},
-{ "start": 1048576, "length": 1048576, "depth": 0, "zero": true, "data": false}]
+[{ "start": 0, "length": 1048576, "depth": 2, "present": true, "zero": false, "data": true, "offset": 327680},
+{ "start": 1048576, "length": 1048576, "depth": 0, "present": false, "zero": true, "data": false}]
 
 Offset          Length          Mapped to       File
 0               0x100000        0x50000         TEST_DIR/PID-base
@@ -220,8 +220,8 @@ read 65536/65536 bytes at offset 5368709120
 1 GiB (0x40000000) bytes not allocated at offset 0 bytes (0x0)
 7 GiB (0x1c0000000) bytes     allocated at offset 1 GiB (0x40000000)
 
-[{ "start": 0, "length": 1073741824, "depth": 1, "zero": true, "data": false},
-{ "start": 1073741824, "length": 7516192768, "depth": 0, "zero": true, "data": false}]
+[{ "start": 0, "length": 1073741824, "depth": 1, "present": false, "zero": true, "data": false},
+{ "start": 1073741824, "length": 7516192768, "depth": 0, "present": true, "zero": true, "data": false}]
 
 === preallocation=metadata ===
 Formatting 'TEST_DIR/PID-base', fmt=qcow2 cluster_size=65536 extended_l2=off compression_type=zlib size=34359738368 lazy_refcounts=off refcount_bits=16
@@ -239,13 +239,13 @@ read 65536/65536 bytes at offset 33285996544
 30 GiB (0x780000000) bytes not allocated at offset 0 bytes (0x0)
 3 GiB (0xc0000000) bytes     allocated at offset 30 GiB (0x780000000)
 
-[{ "start": 0, "length": 32212254720, "depth": 1, "zero": true, "data": false},
-{ "start": 32212254720, "length": 536870912, "depth": 0, "zero": true, "data": false, "offset": 327680},
-{ "start": 32749125632, "length": 536870912, "depth": 0, "zero": true, "data": false, "offset": 537264128},
-{ "start": 33285996544, "length": 536870912, "depth": 0, "zero": true, "data": false, "offset": 1074200576},
-{ "start": 33822867456, "length": 536870912, "depth": 0, "zero": true, "data": false, "offset": 1611137024},
-{ "start": 34359738368, "length": 536870912, "depth": 0, "zero": true, "data": false, "offset": 2148139008},
-{ "start": 34896609280, "length": 536870912, "depth": 0, "zero": true, "data": false, "offset": 2685075456}]
+[{ "start": 0, "length": 32212254720, "depth": 1, "present": false, "zero": true, "data": false},
+{ "start": 32212254720, "length": 536870912, "depth": 0, "present": true, "zero": true, "data": false, "offset": 327680},
+{ "start": 32749125632, "length": 536870912, "depth": 0, "present": true, "zero": true, "data": false, "offset": 537264128},
+{ "start": 33285996544, "length": 536870912, "depth": 0, "present": true, "zero": true, "data": false, "offset": 1074200576},
+{ "start": 33822867456, "length": 536870912, "depth": 0, "present": true, "zero": true, "data": false, "offset": 1611137024},
+{ "start": 34359738368, "length": 536870912, "depth": 0, "present": true, "zero": true, "data": false, "offset": 2148139008},
+{ "start": 34896609280, "length": 536870912, "depth": 0, "present": true, "zero": true, "data": false, "offset": 2685075456}]
 
 === preallocation=falloc ===
 Formatting 'TEST_DIR/PID-base', fmt=qcow2 cluster_size=65536 extended_l2=off compression_type=zlib size=10485760 lazy_refcounts=off refcount_bits=16
@@ -263,8 +263,8 @@ read 65536/65536 bytes at offset 9437184
 5 MiB (0x500000) bytes not allocated at offset 0 bytes (0x0)
 10 MiB (0xa00000) bytes     allocated at offset 5 MiB (0x500000)
 
-[{ "start": 0, "length": 5242880, "depth": 1, "zero": true, "data": false},
-{ "start": 5242880, "length": 10485760, "depth": 0, "zero": false, "data": true, "offset": 327680}]
+[{ "start": 0, "length": 5242880, "depth": 1, "present": false, "zero": true, "data": false},
+{ "start": 5242880, "length": 10485760, "depth": 0, "present": true, "zero": false, "data": true, "offset": 327680}]
 
 === preallocation=full ===
 Formatting 'TEST_DIR/PID-base', fmt=qcow2 cluster_size=65536 extended_l2=off compression_type=zlib size=16777216 lazy_refcounts=off refcount_bits=16
@@ -282,8 +282,8 @@ read 65536/65536 bytes at offset 11534336
 8 MiB (0x800000) bytes not allocated at offset 0 bytes (0x0)
 4 MiB (0x400000) bytes     allocated at offset 8 MiB (0x800000)
 
-[{ "start": 0, "length": 8388608, "depth": 1, "zero": true, "data": false},
-{ "start": 8388608, "length": 4194304, "depth": 0, "zero": false, "data": true, "offset": 327680}]
+[{ "start": 0, "length": 8388608, "depth": 1, "present": false, "zero": true, "data": false},
+{ "start": 8388608, "length": 4194304, "depth": 0, "present": true, "zero": false, "data": true, "offset": 327680}]
 
 === preallocation=off ===
 Formatting 'TEST_DIR/PID-base', fmt=qcow2 cluster_size=65536 extended_l2=off compression_type=zlib size=393216 lazy_refcounts=off refcount_bits=16
@@ -301,9 +301,9 @@ read 65536/65536 bytes at offset 259072
 192 KiB (0x30000) bytes not allocated at offset 0 bytes (0x0)
 320 KiB (0x50000) bytes     allocated at offset 192 KiB (0x30000)
 
-[{ "start": 0, "length": 196608, "depth": 1, "zero": true, "data": false},
-{ "start": 196608, "length": 65536, "depth": 0, "zero": false, "data": true, "offset": 327680},
-{ "start": 262144, "length": 262144, "depth": 0, "zero": true, "data": false}]
+[{ "start": 0, "length": 196608, "depth": 1, "present": false, "zero": true, "data": false},
+{ "start": 196608, "length": 65536, "depth": 0, "present": true, "zero": false, "data": true, "offset": 327680},
+{ "start": 262144, "length": 262144, "depth": 0, "present": true, "zero": true, "data": false}]
 
 === preallocation=off ===
 Formatting 'TEST_DIR/PID-base', fmt=qcow2 cluster_size=65536 extended_l2=off compression_type=zlib size=409600 lazy_refcounts=off refcount_bits=16
@@ -321,8 +321,8 @@ read 65536/65536 bytes at offset 344064
 256 KiB (0x40000) bytes not allocated at offset 0 bytes (0x0)
 256 KiB (0x40000) bytes     allocated at offset 256 KiB (0x40000)
 
-[{ "start": 0, "length": 262144, "depth": 1, "zero": true, "data": false},
-{ "start": 262144, "length": 262144, "depth": 0, "zero": true, "data": false}]
+[{ "start": 0, "length": 262144, "depth": 1, "present": false, "zero": true, "data": false},
+{ "start": 262144, "length": 262144, "depth": 0, "present": true, "zero": true, "data": false}]
 
 === preallocation=off ===
 Formatting 'TEST_DIR/PID-base', fmt=qcow2 cluster_size=65536 extended_l2=off compression_type=zlib size=524288 lazy_refcounts=off refcount_bits=16
@@ -340,6 +340,6 @@ read 65536/65536 bytes at offset 446464
 256 KiB (0x40000) bytes not allocated at offset 0 bytes (0x0)
 244 KiB (0x3d000) bytes     allocated at offset 256 KiB (0x40000)
 
-[{ "start": 0, "length": 262144, "depth": 1, "zero": true, "data": false},
-{ "start": 262144, "length": 249856, "depth": 0, "zero": true, "data": false}]
+[{ "start": 0, "length": 262144, "depth": 1, "present": false, "zero": true, "data": false},
+{ "start": 262144, "length": 249856, "depth": 0, "present": true, "zero": true, "data": false}]