Re: [PATCH 0/3] btrfs: qgroup: Fix the long existing regression of btrfs/153

[David Sterba <dsterba@suse.cz>]

On Thu, Jul 02, 2020 at 08:14:31AM +0800, Qu Wenruo wrote:
> Since commit c6887cd11149 ("Btrfs: don't do nocow check unless we have to"),
> btrfs/153 always fails with early EDQUOT.
> 
> This is caused by the fact that:
> - We always reserved data space for even NODATACOW buffered write
>   This is mostly to improve performance, and not pratical to revert.
> 
> - Btrfs qgroup data and meta reserved space share the same limit
>   So it's not ensured to return EDQUOT just for that data reservation,
>   metadata reservation can also get EDQUOT, means we can't go the same
>   solution as that commit.
> 
> This patchset will solve it by doing extra qgroup space flushing when
> EDQUOT is hit.
> 
> This is a little like what we do in ticketing space reservation system,
> but since there are very limited ways for qgroup to reclaim space,
> currently it's still handled in qgroup realm, not reusing the ticketing
> system yet.
> 
> By this, this patch could solve the btrfs/153 problem, while still keep
> btrfs qgroup space usage under the limit.
> 
> The only cost is, when we're near qgroup limit, we will cause more dirty
> inodes flush and transaction commit, much like what we do when the
> metadata space is near exhausted.
> So the cost should be still acceptable.

This sounds like a reasonable solution to me. Making the behaviour
closer to ticket reservations would probably make it easier to switch
some day.