NAME¶

mdflush - Trace md flush events. Uses Linux eBPF/bcc.

SYNOPSIS¶

mdflush

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 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 bcc.

EXAMPLES¶

Trace md flush events:

# mdflush

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 bcc.

https://github.com/iovisor/bcc

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

OS¶

Linux

AUTHOR¶

Brendan Gregg

SEE ALSO¶

biosnoop(8)