Eran Tromer wrote:
Greetings,
How does one go about tracing *physical* disk I/O on Linux?
Level 1: trace physical I/O requests:
"wrote 4 sectors at offset 533624 on /dev/hda1"
Level 2: report/filter-by PID:
"PID 256 (/bin/foo) wrote 4 sectors at offset 533624 on /dev/hda1"
Level 3: map physical locations back to files:
"PID 256 (/bin/foo) wrote 2048 bytes to inode 535 (/tmp/foo)"
To goal is to understand and reduce disk usage on a loaded system.
It would suffice to get totals per file/process over several seconds
instead of per-event notification.
Level 1 requires a pretty straightforward kernel patch; some are
available (but appear outdated). Totals per device can be obtained via
/proc/stat or 'iostat', so one can inconveniently find totals over a
subset of files by moving those files to a dedicated partition.
Level 2 seems to require finding the process that last accessed a buffer
block, or (easier and nearly as good) the file associated with the
buffer block and some process that has that file open. Note the
difficulty with memory-mapped files.
Level 3 seems to require filesystem hooks or filesystem-specific code.
The following post describes responses to an essentially identical
question regarding Solaris, including one Level 2 solution that's
Solaris-specific.
http://www.netsys.com/sunmgr/1999-01/msg00027.html
BTW, while searching for the above I chanced upon the following the
Linux Trace Toolkit (http://www.opersys.com/LTT/), whose functionality
somewhat overlaps syscalltrack -- it can trace syscalls system-wide,
though it does not appear to handle syscall arguments. OTOH it can trace
scheduling and context switches.
Eran
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