Abhishek Bansal Abhishek Bansal - 5 months ago 30
Linux Question

Calculating Disk Read/Write in Linux with C++

My requirement is to profile current process disk read/write operations with total disk read/write operations (or amount of data read/written). I need to take samples evry second and plot a graph between these two. I need to do this on Linux (Ubuntu 12.10) in c++.

Are there any APIs/Tools available for this task ? I found one tool namely

iotop
but I am not sure how to use this for current process vs system wide usage.

Thank You

Answer

You can read the file /proc/diskstats every second. Each line represents one device.

From kernel's "Documentation/iostat.txt":

Field  1 -- # of reads completed
    This is the total number of reads completed successfully.
Field  2 -- # of reads merged, field 6 -- # of writes merged
    Reads and writes which are adjacent to each other may be merged for
    efficiency.  Thus two 4K reads may become one 8K read before it is
    ultimately handed to the disk, and so it will be counted (and queued)
    as only one I/O.  This field lets you know how often this was done.
Field  3 -- # of sectors read
    This is the total number of sectors read successfully.
Field  4 -- # of milliseconds spent reading
    This is the total number of milliseconds spent by all reads (as
    measured from __make_request() to end_that_request_last()).
Field  5 -- # of writes completed
    This is the total number of writes completed successfully.
Field  7 -- # of sectors written
    This is the total number of sectors written successfully.
Field  8 -- # of milliseconds spent writing
    This is the total number of milliseconds spent by all writes (as
    measured from __make_request() to end_that_request_last()).
Field  9 -- # of I/Os currently in progress
    The only field that should go to zero. Incremented as requests are
    given to appropriate struct request_queue and decremented as they finish.
Field 10 -- # of milliseconds spent doing I/Os
    This field increases so long as field 9 is nonzero.
Field 11 -- weighted # of milliseconds spent doing I/Os
    This field is incremented at each I/O start, I/O completion, I/O
    merge, or read of these stats by the number of I/Os in progress
    (field 9) times the number of milliseconds spent doing I/O since the
    last update of this field.  This can provide an easy measure of both
    I/O completion time and the backlog that may be accumulating.

For each process, you can get use /proc/<pid>/io, which produces something like this:

rchar: 2012
wchar: 0
syscr: 7
syscw: 0
read_bytes: 0
write_bytes: 0
cancelled_write_bytes: 0
  • rchar, wchar: number of bytes read/written.
  • syscr, syscw: number of read/write system calls.
  • read_bytes, write_bytes: number of bytes read/written to storage media.
  • cancelled_write_bytes: from the best of my understanding, caused by calls to "ftruncate" that cancel pending writes to the same file. Probably most often 0.
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