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

[Max Reitz <mreitz@redhat.com>]

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.

Max