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

[Andrey Shinkevich]

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?

Andrey

>> A comment for the option would warn a user that guest writing will work
>> slower for if compress selected.
> 
> I’m not sure whether that’s necessary.  As a user, I would assume that
> enabling compression will always make something slower.
> 
> Max
>