Re: [linux-lvm] exposing snapshot block device

[Zdenek Kabelac <zkabelac@redhat.com>]

Dne 23. 10. 19 v 16:37 Gionatan Danti napsal(a):
> On 23/10/19 14:59, Zdenek Kabelac wrote:
>> Dne 23. 10. 19 v 13:08 Gionatan Danti napsal(a):
>>> Talking about thin snapshot, an obvious performance optimization which 
>>> seems to not be implemented is to skip reading source data when overwriting 
>>> in larger-than-chunksize blocks.
> 
> "For example, consider a completely filled 64k chunk thin volume (with 
> thinpool having ample free space). Snapshotting it and writing a 4k block on 
> origin will obviously cause a read of the original 64k chunk, an in-memory 
> change of the 4k block and a write of the entire modified 64k block to a new 
> location. But writing, say, a 1 MB block should *not* cause the same read on 
> source: after all, the read data will be immediately discarded, overwritten by 
> the changed 1 MB block."
> 
> I would expect that such large-block *thin* snapshot rewrite behavior would 
> not cause a read/modify/write, but it really does.
> 
> Is this a low-hanging fruit or there are more fundamental problem avoiding 
> read/modify/write in this case?

Hi

If you use 1MiB chunksize for thin-pool and you use  'dd' with proper bs size
and you write 'aligned' on 1MiB boundary (be sure you user  directIO, so you 
are not a victim of some page cache flushing...) - there should not be any 
useless read.

If you still do see such read - and you can easily reproduce this with latest 
kernel - report a bug please with your reproducer and results.

Regards

Zdenek