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Add an explanation of the block size units output by prtblknos. 

Suggested by: Ravi Pokala (rpokala@)
This commit is contained in:
Kirk McKusick 2018-02-09 22:23:10 +00:00
parent ff879b0799
commit c97f1d3f37
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tools/diag/prtblknos/README
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@ -27,5 +27,22 @@ 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