.TH mdflush 8  "2018-09-07" "USER COMMANDS"
.SH NAME
mdflush.bt \- Trace md flush events. Uses bpftrace/eBPF.
.SH SYNOPSIS
.B mdflush.bt
.SH 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.
.SH REQUIREMENTS
CONFIG_BPF and bpftrace.
.SH EXAMPLES
.TP
Trace md flush events:
#
.B mdflush.bt
.SH FIELDS
.TP
TIME
Time of the flush event (HH:MM:SS).
.TP
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.
.TP
COMM
The command name for the PID.
.TP
DEVICE
The md device name.
.SH OVERHEAD
Expected to be negligible.
.SH SOURCE
This is from bpftrace.
.IP
https://github.com/iovisor/bpftrace
.PP
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.
.IP
https://github.com/iovisor/bcc
.SH OS
Linux
.SH STABILITY
Unstable - in development.
.SH AUTHOR
Brendan Gregg
.SH SEE ALSO
biosnoop(8)