Re: [PATCH v3 6/6] block/qcow2: use seqcache for compressed writes

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

16.03.2021 15:25, Max Reitz wrote:
> On 15.03.21 15:40, Vladimir Sementsov-Ogievskiy wrote:
>> 15.03.2021 12:58, Max Reitz wrote:
> 
> [...]
> 
>>> The question is whether it really makes sense to even have a seqcache_read() path when in reality it’s probably never accessed.  I mean, besides the fact that it seems based purely on chance whether a read might fetch something from the cache even while we’re writing, in practice I don’t know any case where we’d write to and read from a compressed qcow2 image at the same time.  (I don’t know what you’re doing with the 'compress' filter, though.)
>>>
>>
>> Note, that for user that's not a parallel write and read to the same cluster:
>>
>> 1. user writes cluster A, request succeeded, data is in the cache
>>
>> 2. user writes some other clusters, cache filled, flush started
>>
>> 3. in parallel to [2] user reads cluster A. From the POV of user, cluster A is written already, and should be read successfully
> 
> Yes, but when would that happen?
> 
>> And seqcache_read() gives a simple non-blocking way to support read operation.
> 
> OK, that makes sense.  We’d need to flush the cache before we can read anything from the disk, so we should have a read-from-cache branch here.
> 
>> But rewriting compressed clusters is sensible only when we run real guest on compressed image.. Can it be helpful? Maybe for scenarios with low disk usage ratio..
> 
> I’m not sure, but the point is that rewrites are currently not supported.  The whole compression implementation is mainly tailored towards just writing a complete image (e.g. by qemu-img convert or the backup job), so that’s where my question is coming from: It’s difficult for me to see a currently working use case where you’d read from and write to a compressed image at the same time.
> 

External backup works like the following:

  - start backup sync=none from active disk to temporary disk
  - export temporary disk through nbd
  - external tool reads from nbd export

For this scheme it may make sense to use compression, and we get a use case where compressed reads and writes are used in the same time. Moreover this is possible just now, and no reason to not support it.


-- 
Best regards,
Vladimir