Index: head/tools/diag/prtblknos/README
===================================================================
--- head/tools/diag/prtblknos/README	(revision 329086)
+++ head/tools/diag/prtblknos/README	(revision 329087)
@@ -1,31 +1,48 @@
 # $FreeBSD$
 
 This is a little C-program that can be used to print out the list
 of blocks used by a requested list of inodes.
 
 For example, to list the blocks referenced by your kernel:
 
 guest_12 % df /
 Filesystem      1K-blocks     Used   Avail Capacity  Mounted on
 /dev/gpt/rootfs  20307196 10707336 7975288    57%    /
 
 guest_12 % ls -i /boot/kernel/kernel
 160603 /boot/kernel/kernel
 
 guest_12 % ./prtblknos /dev/gpt/rootfs 160603
 160603:	lbn 0-7 blkno 3217584-3217647
 	lbn 8-11 blkno 3217864-3217895 distance 216
 First-level indirect, blkno 3217896-3217903 distance 0
 	lbn 12-19 blkno 3217904-3217967 distance 8
 	lbn 20-75 blkno 3251816-3252263 distance 33848
 	lbn 76-83 blkno 3252368-3252431 distance 104
 	lbn 84-91 blkno 3252464-3252527 distance 32
 	lbn 92-852 blkno 3252896-3258983 distance 368
 
 Each contiguous range of blocks is printed on a line.
 The distance metric is the size of the gap from the end of the
 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