xperf disk io graph about flushes item

[wenporu]

hi all,
I have a question about an xperf diskI/O's item.
whats meaning of flushes item of the diskI/O?
Does it mean that write the log file to disk or other issue?

there is its screenshot:

Why only my driver c has the flushes operation? (as the screenshot of red line)
my driver g is running iometer, but it has not flushes.

thank you!

 

[cluberti]

That's disk cache flushes to the drive - iometer tests a drive by measuring I/O to the disk subsystem itself, and bypasses the cache entirely (as that would skew the results entirely). What you must understand is that some I/O that is written to the disk is actually written to the cache first, because writing to cache is generally quicker than actually putting data to disk - especially when a disk is busy. It can then be written to the disk later, when the disk is not as busy, thus keeping OS performance from getting bogged down due to disk transfer speed. This can be controlled by the application writing the data to disk, or the disk controller itself. What you're seeing is normal behavior, on both drives.