2011-05-19 17:00, Jim Klimov пишет:
I am not sure you can monitor actual mechanical seeks short
of debugging and interrogating the HDD firmware - because
it is the last responsible logic in the chain of caching,
queuing and issuing actual commands to the disk heads.
For example, a long logical IO spanning several cylinders
would probably be seen by the OS as one IO with many MBs
of transfer, but would break down into track-to-track seeks
in disk's reality.
i.e. when iostat reports "500" read ops, does that translate to
500 seeks + 1 read per seek, or 50 seeks + 10 reads, etc? Thanks!
First I wanted to assume that this would map to "at least
500 seeks and 1 read per seek". But...
Actually, I am not sure there is any one good translation
of logical IOPS as seen by ZFS, storage driver or iostat,
into mechanical IOPS.
Logical IOs issued by ZFS may be grouped in one mechanical
IO by the HDD's firmware, if these IOs happen to be backed
by blocks on the same track (and they are issued in a rapid
enough succession as to be natively/taggedly queued as one
IO). I wrote of an opposite situation above.
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