mdflush.bt(8) System Manager's Manual mdflush.bt(8)
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 ker-
nel 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.bt(8)
USER COMMANDS 2018-09-07 mdflush.bt(8)
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