Gilad
One more input, which is important (imho) - this is a performance
measurement / system tuning objective, not a system accounting
application - i.e.we are not trying to charge someone
for io usage ( i think they used to do that once upon a time....).
10x
dL
Gilad Ben-Yossef wrote:
Danny Lieberman wrote:
I have a simply question, namely "How many disk block reads and
writes is a process doing over a given period of time, and what is
the minimum, maximum and average stats"
This seemingly trivial question (which can easily be answered in a
Windows operating system using the system performance monitor)
appears to be a gray area at best.
I know there is work on io-accounting in the new versions of the
kernel -- but a) I dont have the luxury of updating the kernel and b)
these are stats I would expect to see in user space
like vmstat or top in any version of the kernel.
Still waiting for an answer.
I am sure one of the people on the list will know the answer.
The reason there is no good answer to your question is because it
seems on it's face to be very well defined, but it really isn't. I'll
try to explain why:
In Linux processes do not talk to block devices directly (unless using
direct IO calls, which is rare). Processes make system calls that call
on the file system drivers that call on the buffer cache which in turn
uses the block device drivers to write/read from actual disk hardware.
The problem is, that all this does not happen in a synchronious manner
- the process can make a write system call that will put data in the
buffer cache and the kernel will decide to flush those buffer to disk
3.5 seconds later (perhaps even after the proccess has ended!), along
with half a dozen other buffers from other processes. Do you charge
the proccess for this IO activity and how?
In a similar manner: it is possible that a write will go to cache, a
second write (from same or different proccess) will update the buffer
in the cace and only the end buffer will be written to disk. When the
buffer will written to disk at the end - who do you charge for the IO
- the first proccess, the second proccess, both?
Since the correct answer to these and similar question is completly
situation dependendant, a general tool that does what you want does
not exist. If you will describe in detail your need we can advise on a
good way to acheive it.
Hope this helps,
Gilad
--
Danny Lieberman
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