| 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 |
| 03:15:11 488 xfsaild/md0 md0 |
| 03:15:11 488 xfsaild/md0 md0 |
| 03:15:11 488 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. |
