Re: [PATCH, RFC] xfs: use per-AG reservations for the finobt

[Christoph Hellwig <hch@lst.de>]

On Tue, Jan 03, 2017 at 11:24:26AM -0800, Darrick J. Wong wrote:
> ...and so here we calculate the number of blocks needed to store the
> maximum number of finobt records possible for an AG.  IIRC, each *inobt
> record refers to a single chunk of 64 inodes (or at least a theoretical
> chunk in the spinodes=1 case), so I think we can reduce the reservation
> to...
> 
> nr = m_sb.sb_agblocks * m_sb.sb_inopblock / XFS_INODES_PER_CHUNK;
> return xfs_inobt_calc_size(mp, nr);
> 
> ...right?

Yes, that should reduce the reservation quite a bit.

> This requires us to traverse all the blocks in the finobt at mount time,
> which isn't necessarily quick.  For refcount/rmap we cache the number of
> tree blocks in the AGF to speed this up... but it was easy to sneak that
> into the disk format. :)

But for finobt it's too late to do that without another incompatible
feature flag.

> For finobt I wonder if one could defer the block counting work to a
> separate thread if the AG has enough free blocks to cover, say, 10x the
> maximum reservation?  Though that could be racy and maybe finobts are
> small enough that the impact on mount time is low anyway?

Usually they are small.  And if they aren't - well that's life.

I don't think anync counting for a reservation is a good idea.  If we
see a problem with the time needed to count in practice we'll have to
keep a count an introduce a feature flag.

> There's also the unsolved problem of what happens if we mount and find
> agf_freeblks < (sum(ask) - sum(used)) -- right now we eat that state and
> hope that we don't later ENOSPC and crash.

Yes.  Which is exactly the situation we would have without this
patch anyway..

> But as for retroactively adding AG reservations for an existing tree, I
> guess we'll have to come up with a strategy for dealing with
> insufficient free blocks.  I suppose one could try to use xfs_fsr to
> move large contiguous extents to a less full AG, if there are any...

Eww.  We could just fall back to the old code before this patch,
which would then eventually shut down..