NAME

       mdflush.bt - Trace md flush events. Uses bpftrace/eBPF.

SYNOPSIS

       mdflush.bt

DESCRIPTION

       This tool traces flush events by md, the Linux multiple device driver (software RAID). The
       timestamp and md device for the flush are printed.  Knowing when these flushes happen  can
       be useful for correlation with unexplained spikes in disk latency.

       This works by tracing the kernel md_flush_request() function using kernel dynamic tracing,
       and will need updating to match any changes to this function.

       Note that the flushes themselves are likely to originate from higher  in  the  I/O  stack,
       such as from the file systems.

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

REQUIREMENTS

       CONFIG_BPF and bpftrace.

EXAMPLES

       Trace md flush events:
              # mdflush.bt

FIELDS

       TIME   Time of the flush event (HH:MM:SS).

       PID    The  process  ID  that  was on-CPU when the event was issued. This may identify the
              cause of the flush (eg, the "sync" command),  but  will  often  identify  a  kernel
              worker thread that was managing I/O.

       COMM   The command name for the PID.

       DEVICE The md device name.

OVERHEAD

       Expected to be negligible.

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.

       This is a bpftrace version of the bcc tool of the same name. The bcc tool may provide more
       options and customizations.

              https://github.com/iovisor/bcc

OS

       Linux

STABILITY

       Unstable - in development.

AUTHOR

       Brendan Gregg

SEE ALSO

       biosnoop(8)