Re: [PATCH v5 1/4] block: support compressed write at generic layer

[Andrey Shinkevich]

On 24/10/2019 16:48, Max Reitz wrote:
> On 24.10.19 14:56, Andrey Shinkevich wrote:
>>
>>
>> On 24/10/2019 12:34, Max Reitz wrote:
>>> On 22.10.19 15:53, Andrey Shinkevich wrote:
>>>
>>> [...]
>>>
>>>> If the support of COW for compressed writes is found feasible, will it
>>>> make a sense to implement? Then this series will follow.
>>>
>>> Hm, what exactly do you mean by support of COW for compressed writes?
>>>
>>
>> I spoke in terms of the commit message with the following ID:
>>
>> b0b6862e5e1a1394e0ab3d5da94ba8b0da8664e2
>>
>> "qcow2: Fail write_compressed when overwriting data"
>>
>> "...qcow2_write_compressed() doesn't perform COW as it would have to do..."
>>
>> So, I suggest that we implement writing compressed data to the allocated
>> clusters rather than qcow2_alloc_compressed_cluster_offset() returns the
>> error. Particularly, when it comes to NBD server connection failure for
>> writhing a compressed cluster, it may not be rewritten after the
>> connection is restored.
>> Are there any issues with that implementation idea?
> 
> Well, the COW in that commit is meant differently, because it refers to
> the COW that’s required when writing to a cluster shared by an internal
> snapshot.
> 
> OTOH, you could say that all compressed writes to a cluster that is
> already allocated would need to do COW because we’d always have to fully
> rewrite that cluster in an RMW cycle.
> 
> I don’t see how letting qcow2_alloc_compressed_cluster_offset() use the
> existing cluster would solve the problem, though.  You’d generally need
> to allocate a new cluster; or attempt to reuse the existing space in a
> compressed cluster.
> 
> Max
> 

The idea was to avoid the following error:

$ ./qemu-img create -f qcow2 -o size=10M ./image.qcow2
Formatting './image.qcow2', fmt=qcow2 size=10485760 cluster_size=65536 
lazy_refcounts=off refcount_bits=16
[andrey@dhcp-172-16-25-136 qemu]$ git diff --name-only
nbd/server.c
$ ./qemu-io -c "write -P 0x12 0 64k" ./image.qcow2
wrote 65536/65536 bytes at offset 0
64 KiB, 1 ops; 0.0152 sec (4.101 MiB/sec and 65.6082 ops/sec)
$ ./qemu-io -c "write -c -P 0x12 0 64k" ./image.qcow2
write failed: Input/output error

If it incurs a high cost, we can choose the filter option as discussed 
above.

Andrey