NAME

       writeback.bt - Trace file system writeback events with details. Uses bpftrace/eBPF.

SYNOPSIS

       writeback.bt

DESCRIPTION

       This  traces  when  file system dirtied pages are flushed to disk by kernel writeback, and
       prints details including when the event occurred, and the duration of the event. This  can
       be  useful  for correlating these times with other performance problems, and if there is a
       match, it would be a clue that the problem may be caused by  writeback.  How  quickly  the
       kernel does writeback can be tuned: see the kernel docs, eg, vm.dirty_writeback_centisecs.

       This          uses          the          tracepoint:writeback:writeback_start          and
       tracepoint:writeback:writeback_written tracepoints.

       Since this uses BPF, only the root user can use this tool.

REQUIREMENTS

       CONFIG_BPF and bpftrace.

EXAMPLES

       Trace all writeback events with timestamps and latency details:
              # writeback.bt

FIELDS

       TIME   Time that the writeback event completed, in %H:%M:%S format.

       DEVICE Device name in major:minor number format.

       PAGES  Pages written during writeback.

       REASON Reason for the  writeback  event.  This  may  be  "background",  "vmscan",  "sync",
              "periodic", etc.

       ms     Duration of the writeback event in milliseconds.

OVERHEAD

       Since  writeback events are expected to be infrequent (<10/sec), the overhead of this tool
       is expected to be negligible (near 0%).

SOURCE

       This is from bpftrace.

              https://github.com/iovisor/bpftrace

       Also look in the bpftrace distribution  for  a  companion  _examples.txt  file  containing
       example usage, output, and commentary for this tool.

OS

       Linux

STABILITY

       Unstable - in development.

AUTHOR

       Brendan Gregg

SEE ALSO

       biosnoop(8)