Author: mckusick
Date: Fri Feb  9 22:23:10 2018
New Revision: 329087
URL: https://svnweb.freebsd.org/changeset/base/329087

Log:
  Add an explanation of the block size units output by prtblknos.
  
  Suggested by: Ravi Pokala (rpokala@)

Modified:
  head/tools/diag/prtblknos/README

Modified: head/tools/diag/prtblknos/README
==============================================================================
--- head/tools/diag/prtblknos/README    Fri Feb  9 21:51:08 2018        
(r329086)
+++ head/tools/diag/prtblknos/README    Fri Feb  9 22:23:10 2018        
(r329087)
@@ -27,5 +27,22 @@ The distance metric is the size of the gap from the en
 previous set of blocks to the beginning of the next set of blocks.
 Short distances are desirable.
 
+The logical block numbers (lbn above) describe filesystem-block
+size blocks which by today's default is 32Kb.
+
+The physical block numbers (blkno above) describe the smallest piece
+of disk space that an inode can reference which is a filesystem
+fragment. Since the default size for fragments today is 4Kb, the
+physical block numbers reference 4Kb blocks. The distances listed
+above are also in physical block size units. Thus a distance of 8
+means a separation of just one 32Kb block. For example lbn 11, ends
+at 3217895, the 32Kb first level indirect immediately follows it at
+3217896-3217903, and lbn 12 starts distance 8 after lbn 11 immediately
+after the indirect block at 3217904.
+
+If you were to create a filesystem with the fragment size equal to
+half the block size (say 32Kb blocks with 16Kb fragments) then the
+physical block numbers listed would be 16Kb in size.
+
        Marshall Kirk McKusick
        January 19, 2018
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