On Tue, Aug 14, 2012 at 3:22 PM, Josef Bacik [off-list ref] wrote:

Swinging this pendulum back the other way.  We've been allocating chunks up
to 2% of the disk no matter how much we actually have allocated.  So instead
fix this calculation to only allocate chunks if we have more than 80% of the
space available allocated.  Please test this as it will likely cause all
sorts of ENOSPC problems to pop up suddenly.  Thanks,

Signed-off-by: Josef Bacik <redacted>

I've been testing this patch with my multiple rsync test (On a 3.5.1
kernel merged with for-linus).

I tested without compression, and with lzo compression, and I haven't
run into any ENOSPC issues.  I still have ENOSPC issues with zlib,
with or without this patch.

I made a series of  runs with and without this patch (on an
uncompressed, newly formatted partition), and some of the results were
not what I anticipated.

1) I found that *MORE* metadata space was being allocated with this
patch than when using an unpatched baseline kernel.  The total
allocated space was exactly the same in each run (I saw a slight
variation in the amount of used Metadata).

On the unpatched baseline kernel, at the end of the run, the 'btrfs fi
df' command would show:

# btrfs fi df /mnt/benchmark/
Data: total=10.01GB, used=6.99GB
System: total=4.00MB, used=4.00KB
Metadata: total=776.00MB, used=481.38MB

With this patch applied, the 'btrfs fi df' command would show:

# btrfs fi df /mnt/benchmark/
Data: total=10.01GB, used=6.99GB
System: total=4.00MB, used=4.00KB
Metadata: total=1.01GB, used=480.94MB


2)  The multiple rsync's would run significantly faster with the patched kernel.

Unpatched baseline kernel:  Time to run 7 rysncs:  348.3 sec (+/- 9.7 sec)
Patched kernel: Time to run 7 rsyncs:  316.6 sec (+/- 6.5 sec)

Perhaps the extra allocated metadata space made things run better, or
perhaps something else was going on.