Zach Brown wrote:
Can you clarify what you mean above with an example? I don't really
follow.
Sure, take 'tar' as an example. It'll read files in the order that
their names are returned from directory listing. This can produce bad
IO patterns because the order in which the file names are returned
doesn't match the order of the file's blocks on disk. (htree, I'm
looking at you!)
People have noticed that tar-like loads can be sped up greatly just by
sorting the files by their inode number as returned by stat(), never
mind the file blocks themselves. One example of this is Chris Mason's
'acp'.
http://oss.oracle.com/~mason/acp/
The logical extension of that is to use FIBMAP to find the order of file
blocks on disk and then doing IO on blocks in sorted order. It'd take
work to write an app that does this reliably, sure.
In this use the application doesn't actually care what the absolute
numbers are. It cares about their ordering. File systems would be able
to chose whatever scheme they wanted for the actual values of the
results from a FIBMAP-alike as long as the sorting resulted in the right
IO patterns.
Arguing that this use is significant enough to justify an addition to
the file system API is a stretch. I'm just sharing the observation.
- z
I use FIBMAP support for a few different things.
The first is to exactly the case that you describe above where we can
use the first block of a file extracted by FIBMAP to produce an optimal
sorting for the read order. My testing showed that the cost of the
extra fibmap was not too high compared to the speedup, but it was not a
huge gain over the speedup gained when the read was done in inode sorted
order.
The second use case is to look at the physical layout of blocks on disk
for a specific file, use Mark Lord's write_long patches to inject a disk
error and then read that file to make sure that we are handling disk IO
errors correctly. A bit obscure, but really quite useful.
We have also used FIBMAP a few times to try and map an observed IO error
back to a file. Really slow and painful to do, but should work on any
file system when a better method is not supported.
ric
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