[PULL 02/17] nbd: server: Report holes for raw images

[Eric Blake]

From: Nir Soffer <nirsof@gmail.com>

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>
---
 nbd/server.c               | 4 ++--
 tests/qemu-iotests/241.out | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/nbd/server.c b/nbd/server.c
index 7229f487d296..86a44a9b41c1 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -2087,8 +2087,8 @@ static int blockstatus_to_extents(BlockDriverState *bs, 
uint64_t offset,
             return ret;
         }

-        flags = (ret & BDRV_BLOCK_ALLOCATED ? 0 : NBD_STATE_HOLE) |
-                (ret & BDRV_BLOCK_ZERO      ? NBD_STATE_ZERO : 0);
+        flags = (ret & BDRV_BLOCK_DATA ? 0 : NBD_STATE_HOLE) |
+                (ret & BDRV_BLOCK_ZERO ? NBD_STATE_ZERO : 0);

         if (nbd_extent_array_add(ea, num, flags) < 0) {
             return 0;
diff --git a/tests/qemu-iotests/241.out b/tests/qemu-iotests/241.out
index 75f9f465e522..3f8c173cc82d 100644
--- a/tests/qemu-iotests/241.out
+++ b/tests/qemu-iotests/241.out
@@ -5,7 +5,7 @@ QA output created by 241
   size:  1024
   min block: 1
 [{ "start": 0, "length": 1000, "depth": 0, "zero": false, "data": true, 
"offset": OFFSET},
-{ "start": 1000, "length": 24, "depth": 0, "zero": true, "data": true, 
"offset": OFFSET}]
+{ "start": 1000, "length": 24, "depth": 0, "zero": true, "data": false, 
"offset": OFFSET}]
 1 KiB (0x400) bytes     allocated at offset 0 bytes (0x0)

 === Exporting unaligned raw image, forced server sector alignment ===
@@ -23,6 +23,6 @@ WARNING: Image format was not specified for 'TEST_DIR/t.raw' 
and probing guessed
   size:  1024
   min block: 1
 [{ "start": 0, "length": 1000, "depth": 0, "zero": false, "data": true, 
"offset": OFFSET},
-{ "start": 1000, "length": 24, "depth": 0, "zero": true, "data": true, 
"offset": OFFSET}]
+{ "start": 1000, "length": 24, "depth": 0, "zero": true, "data": false, 
"offset": OFFSET}]
 1 KiB (0x400) bytes     allocated at offset 0 bytes (0x0)
 *** done
-- 
2.30.1