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). > Then, I wrote a simple utility > <https://gist.github.com/992364ceca984d3f14099ec94aaacd9d> to make > TONS of > holes in a filesystem by calling fallocate() to punch holes in a file > that is almost as large as the volume size. > > I created an XFS filesystem image by the following steps: > 1. fallocate -l 80G /mnt/disk2/xfs > 2. mkfs.xfs -f -d agcount=1 /mnt/disk2/xfs > > Then I created a large file by fallocate: > fallocate -l 85823746048 /mnt/test/abc > > which left only 4 blocks available in the volume finally: > /dev/loop0 20961280 20961276 4 100% /mnt/test > > The result of xfs_bmap against /mnt/test/abc: > /mnt/test/abc: > EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL FLAGS > 0: [0..167624503]: 83000..167707503 0 (83000..167707503) 167624504 10000 > > After that, I used the hole-punching utility above to create holes on > the files, and captured the output of kmsg. > > 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... > I'd suggest to use a combination of xfs_db and tracepoints/xfsstats to identify what's happening in your test sequence. E.g., unmount and use xfs_db to identify the state of the free space btree(s) before and after various points of your test. See [1] for examples of how to use xfs_db to explore on-disk data structures. See 'man trace-cmd' to work with tracepoints and /proc/fs/xfs/stats (and /proc/sys/fs/xfs/stats_clear) to view runtime statistics (which I believe already includes the number of btree splits). Brian [1] http://xfs.org/docs/xfsdocs-xml-dev/XFS_Filesystem_Structure//tmp/en-US/html/index.html > _______________________________________________ > xfs mailing list > xfs@xxxxxxxxxxx > http://oss.sgi.com/mailman/listinfo/xfs _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
