Re: [PATCH] Btrfs: change how we calculate the global block rsv

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, May 25, 2016 at 11:33:59PM +0200, David Sterba wrote:
> On Wed, May 25, 2016 at 10:51:34AM -0400, Josef Bacik wrote:
> > Traditionally we've calculated the global block rsv by guessing how much of the
> > metadata used amount was the extent tree, and then taking the data size and
> > figuring out how large the csum tree would have to be to hold that much data.
> > 
> > This is imprecise and falls down on MIXED file systems as we can't trust the
> > data used amount.  This resulted in failures for xfstests generic/333 because it
> > creates lots of clones, which explodes out the extent tree.  Our global reserve
> > calculations were woefully inaccurate in this case which meant we got into a
> > situation where we did not have enough reserved to do our work.
> > 
> > We know we only use the global block rsv for the extent, csum, and root trees,
> > so just get the bytes used for these trees and use that as the basis of our
> > global reserve.  Since these are not reference counted trees the bytes_used
> > value will be accurate.  This fixed the transaction aborts seen with
> > generic/333.  Thanks,
> 
> The abort is gone, the fstest fails with
> 
> generic/333              - output mismatch (see results//generic/333.out.bad)
>     --- tests/generic/333.out   2016-04-08 11:50:13.000000000 +0200
>     +++ results//generic/333.out.bad  2016-05-25 23:30:50.965956265 +0200
>     @@ -2,3 +2,4 @@
>      Format and mount
>      Initialize file
>      Snapshot a file undergoing directio rewrite
>     +touch: cannot touch '/mnt/scratch/test-333/finished': No space left on device

Yeah, the finished flag file should be in $TEST_DIR, not $SCRATCH_MNT.

Patches coming...

--D

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