On Thu, Jul 07, 2016 at 07:01:35PM +0800, Kaho Ng wrote:
> I am trying to investigate how freelist allocator in xfs interacts
> with freespace B+Tree allocator.
> First I prepared a patch
> <https://gist.github.com/22ffca35929e67c08759b057779b7566> on
> linux-source/fs/xfs/libxfs/xfs_alloc.c to print debugging messages
> (The kernel version used is linux-3.10.0-327.22.2.el7).
......
> When reading the log output
> <https://gist.github.com/890076405e1c13c0a952a579e25e6afe> , I
> realised that there is no B+Tree split
> triggered by xfs_alloc_fix_freelist() when calling xfs_free_extent().
> Isn't B+Tree split possible in by-size B+Tree even when truncating a
> longer freespace record to shorter one? But what I found in the log is
> only a few tree shrinks... And when reading the source code of
> freespace allocator I found that a B+Tree growth in this case is
> impossible at least...
args->isfl doesn't mean what you think it means.
args->isfl is only set when moving blocks from the freespace btree
to the AGFL, which only occurs when a previous operation allocated a
new freespace btree block and depleted the current freelist. i.e.
"AG Free List" != "AG freespace btree" - they are different
structures on disk...
And when you consider that a freelist refill can only remove records
from the the freespace btree, it's should be clear that a btree
split won't occur during a freelist refill...
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
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