Accuracy of disk statistics IO counter

[Mark Seger <Mark.Seger@hp.com>]

Awhile back I had suggested moving the place in the block driver logic 
where stats get updated to more accurately reflect what was happening at 
the time they were actually sent to the drivers and I believe they were 
indeed made in the 2.6.15 timeframe.  I've recently started taking 
closer look at the numbers and while the sector counts look correct I'm 
not sure I'd agree with the number of I/Os being reported and believe 
they can be off by maybe 15% or more.

Specifically, I wrote a 1GB file with a blocksize of 1MB, which would 
result in 1000 writes at the application level.  What I believe then 
happens is that each write turns into 8 128KB requests to the driver, 
which should result in 8000 actual writes.  Toss in metadata operations 
and who knows what else and the actual number should be a little 
higher.  What I've see after repeating the tests a number of times on 
2.6.16-27 is numbers ranging from 6800-7000 writes which feels like a 
big enough difference to at least point out.

-mark


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