Re: [PATCH RFC 0/3] Introduce per-profile available space array to avoid over-confident can_overcommit()

[Josef Bacik <josef@toxicpanda.com>]

On 12/25/19 8:39 AM, Qu Wenruo wrote:
> There are several bug reports of ENOSPC error in
> btrfs_run_delalloc_range().
> 
> With some extra info from one reporter, it turns out that
> can_overcommit() is using a wrong way to calculate allocatable metadata
> space.
> 
> The most typical case would look like:
>    devid 1 unallocated:	1G
>    devid 2 unallocated:  10G
>    metadata profile:	RAID1
> 
> In above case, we can at most allocate 1G chunk for metadata, due to
> unbalanced disk free space.
> But current can_overcommit() uses factor based calculation, which never
> consider the disk free space balance.
> 
> 
> To address this problem, here comes the per-profile available space
> array, which gets updated every time a chunk get allocated/removed or a
> device get grown or shrunk.
> 
> This provides a quick way for hotter place like can_overcommit() to grab
> an estimation on how many bytes it can over-commit.
> 
> The per-profile available space calculation tries to keep the behavior
> of chunk allocator, thus it can handle uneven disks pretty well.
> 
> The RFC tag is here because I'm not yet confident enough about the
> implementation.
> I'm not sure this is the proper to go, or just a over-engineered mess.
> 

In general I like the approach, however re-doing the whole calculation once we 
add or remove a chunk seems like overkill.  Can we get away with just doing the 
big expensive calculation on mount, and then adjust available up and down as we 
add and remove chunks?  We know the chunk allocator is not going to allow us to 
allocate more chunks than our smallest device, so we can be sure we're never 
going to go negative, and removing chunks is easy again because of the same reason.

Other than that the approach seems sound to me.  Thanks,

Josef