How to tell how fragmented the free space is on an XFS filesystem?

On an XFS filesystem, containing a large number of small files (i.e. <= 4KiB in size), the adding and deleting of files can cause the free space to get fragmented.  This fragmentation can cause a false "out of space" message despite there being a significant amount of free space available.

Further, a severely fragmented filesystem can cause the inability to create new files as outlined in 7014318 - Error: "no space available" on XFS filesystem

Monitor your Allocation Groups (AGs) periodically -- starting with a baseline measurement and comparing periodically.

If you're not already familiar with xfs_info, you will want to run this first against a mounted XFS filesystem.  It will return data as follows:

meta-data=/dev/evms/csm/fence4   isize=256    agcount=288, agsize=8388607 blks
         =                       sectsz=512   attr=0
data     =                       bsize=4096   blocks=2409365488, imaxpct=25
         =                       sunit=0      swidth=0 blks, unwritten=1
naming   =version 2              bsize=4096
log      =internal               bsize=4096   blocks=32512, version=1
         =                       sectsz=512   sunit=0 blks, lazy-count=0
realtime =none                   extsz=4096   blocks=0, rtextents=0

Of interest for our purposes is:

•  the "agcount" which is 288 AGs -- numbered 0 - 287
• the volume in question is right after "metadata=", or /dev/evms/csm/fence4

Knowing how many AGs we have, one can manually run a simple command line to generate a report on each AG and list it out to a file.  The following is such a command line that will report on each AG to a separate file:

for AGNO in `seq 0 287`;do /usr/sbin/xfs_db -r -c "freesp -s -a $AGNO" /dev/evms/csm/fence4 1>ag${AGNO}.txt

where:

• the "287" is the agcount value
• the "/dev/evms/csm/fence4" is the XFS filesystem
  
• ag${AGNO}.txt is the file capturing the output.
  

After running the above, you will get a number of ag*.txt files.  Looking into each of them you will see a histogram of contiguous free blocks.  Here is an example of an AG in fairly good shape:

   from      to extents  blocks    pct
      1       1       6       6   0.00
      8      15   31220  467872  26.33
1048576 2097151       1 1308895  73.67
total free extents 31227
total free blocks 1776773
average free extent size 56.8986

It shows there are:
   - only 6, 1 block "orphans" -- meaning there are no other free blocks around each block.
   - there are 31220 clusters of 8-15 contiguous blocks
   - there is 1 cluster of 1048576-2097151 contiguous blocks

Conversely, here is an AG that is greatly fragmented:

   from      to extents  blocks    pct
      1       1  979870  979870  68.79
      2       3  205140  444668  31.21
total free extents 1185010
total free blocks 1424538
average free extent size 1.20213

This shows that 68.79% of the free space on the filesystem is made up of only single blocks; and 31.21% is made up of 2-3 block clusters.