Re: [PATCH v2 3/4] qapi: blockdev-backup: add discard-source parameter

[Fiona Ebner <f.ebner@proxmox.com>]

Am 25.01.24 um 18:22 schrieb Vladimir Sementsov-Ogievskiy:
> 
> Hmm. Taking maximum is not optimal for usual case without
> discard-source: user may want to work in smaller granularity than
> source, to save disk space.
> 
> In case with discarding we have two possibilities:
> 
> - either take larger granularity for the whole process like you propose
> (but this will need and option for CBW?)
> - or, fix discarding bitmap in CBW to work like normal discard: it
> should be aligned down. This will lead actually to discard-source option
> doing nothing..
> 
> ==
> But why do you want fleecing image with larger granularity? Is that a
> real case or just experimenting? Still we should fix assertion anyway.
> 

Yes, it's a real use case. We do support different storage types and
want to allow users to place the fleecing image on a different storage
than the original image for flexibility.

I ran into the issue when backing up to a target with 1 MiB cluster_size
while using a fleecing image on RBD (which has 4 MiB cluster_size by
default).

In theory, I guess I could look into querying the cluster_size of the
backup target and trying to allocate the fleecing image with a small
enough cluster_size. But not sure if that would work on all storage
combinations, and would require teaching our storage plugin API (which
also supports third-party plugins) to perform allocation with a specific
cluster size. So not an ideal solution for us.

> I think:
> 
> 1. fix discarding bitmap to make aligning-down (will do that for v3)
> 

Thanks!

> 2. if we need another logic for block_copy_calculate_cluster_size() it
> should be an option. May be explicit "copy-cluster-size" or
> "granularity" option for CBW driver and for backup job. And we'll just
> check that given cluster-size is power of two >= target_size.
> 

I'll try to implement point 2. That should resolve the issue for our use
case.

Best Regards,
Fiona