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Demonstrations of mdflush, the Linux eBPF/bcc version.

The mdflush tool traces flushes at the md driver level, and prints details

including the time of the flush:

# ./mdflush

Tracing md flush requests... Hit Ctrl-C to end.

TIME PID COMM DEVICE

03:13:49 16770 sync md0

03:14:08 16864 sync md0

03:14:49 496 kworker/1:0H md0

03:14:49 488 xfsaild/md0 md0

03:14:54 488 xfsaild/md0 md0

03:15:00 488 xfsaild/md0 md0

03:15:02 85 kswapd0 md0

03:15:02 488 xfsaild/md0 md0

03:15:05 488 xfsaild/md0 md0

03:15:08 488 xfsaild/md0 md0

03:15:10 488 xfsaild/md0 md0

03:15:11 488 xfsaild/md0 md0

03:15:12 488 xfsaild/md0 md0

03:15:13 488 xfsaild/md0 md0

03:15:15 488 xfsaild/md0 md0

03:15:19 496 kworker/1:0H md0

03:15:49 496 kworker/1:0H md0

03:15:55 18840 sync md0

03:16:49 496 kworker/1:0H md0

03:17:19 496 kworker/1:0H md0

03:20:19 496 kworker/1:0H md0

03:21:19 496 kworker/1:0H md0

03:21:49 496 kworker/1:0H md0

03:25:19 496 kworker/1:0H md0

[...]

This can be useful for correlation with latency outliers or spikes in disk

latency, as measured using another tool (eg, system monitoring). If spikes in

disk latency often coincide with md flush events, then it would make flushing

a target for tuning.

Note that the flush events are likely to originate from higher in the I/O

stack, such as from file systems. This traces md processing them, and the

timestamp corresponds with when md began to issue the flush to disks.