On 20.10.19 22:37, Andrey Shinkevich wrote: > To inform the block layer about writing all the data compressed, we > introduce the 'compress' command line option. Based on that option, the > written data will be aligned by the cluster size at the generic layer. > > Suggested-by: Roman Kagan <[email protected]> > Suggested-by: Vladimir Sementsov-Ogievskiy <[email protected]> > Signed-off-by: Andrey Shinkevich <[email protected]> > Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]> > --- > block.c | 20 +++++++++++++++++++- > block/io.c | 13 +++++++++---- > block/qcow2.c | 4 ++++ > blockdev.c | 9 ++++++++- > include/block/block.h | 1 + > include/block/block_int.h | 2 ++ > qapi/block-core.json | 5 ++++- > qemu-options.hx | 6 ++++-- > 8 files changed, 51 insertions(+), 9 deletions(-)
The problem with compression is that there are such tight constraints on
it that it can really only work for very defined use cases. Those
constraints are:
- Only write whole clusters,
- Clusters can be written to only once.
The first point is addressed in this patch by setting request_alignment.
But I don’t see how the second one can be addressed. Well, maybe by
allowing it in all drivers that support compression. But if I just look
at qcow2, that isn’t going to be trivial: You need to allocate a
completely new cluster where you write the data (in case it grows), and
thus you leave behind a hole, which kind of defeats the purpose of
compression.
(For demonstration:
$ ./qemu-img create -f qcow2 test.qcow2 64M
Formatting 'test.qcow2', fmt=qcow2 size=67108864 cluster_size=65536
lazy_refcounts=off refcount_bits=16
$ x86_64-softmmu/qemu-system-x86_64 \
-blockdev "{'node-name': 'drv0', 'driver': 'qcow2',
'compress': true,
'file': {'driver': 'file', 'filename': 'test.qcow2'}}" \
-monitor stdio
QEMU 4.1.50 monitor - type 'help' for more information
(qemu) qemu-io drv0 "write -P 42 0 64k"
wrote 65536/65536 bytes at offset 0
64 KiB, 1 ops; 00.02 sec (4.055 MiB/sec and 64.8793 ops/sec)
(qemu) qemu-io drv0 "write -P 23 0 64k"
write failed: Input/output error
)
Compression really only works when you fully write all of an image
exactly once; i.e. as the qemu-img convert or as a backup target. For
both cases we already have a compression option. So I’m wondering where
this new option is really useful.
(You do add a test for stream, but I don’t know whether that’s really a
good example, see my response there.)
Max
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