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 _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
