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Demonstrations of biosnoop, the Linux eBPF/bcc version.
biosnoop traces block device I/O (disk I/O), and prints a line of output
per I/O. Example:
# ./biosnoop
TIME(s) COMM PID DISK T SECTOR BYTES LAT(ms)
0.000004001 supervise 1950 xvda1 W 13092560 4096 0.74
0.000178002 supervise 1950 xvda1 W 13092432 4096 0.61
0.001469001 supervise 1956 xvda1 W 13092440 4096 1.24
0.001588002 supervise 1956 xvda1 W 13115128 4096 1.09
1.022346001 supervise 1950 xvda1 W 13115272 4096 0.98
1.022568002 supervise 1950 xvda1 W 13188496 4096 0.93
1.023534000 supervise 1956 xvda1 W 13188520 4096 0.79
1.023585003 supervise 1956 xvda1 W 13189512 4096 0.60
2.003920000 xfsaild/md0 456 xvdc W 62901512 8192 0.23
2.003931001 xfsaild/md0 456 xvdb W 62901513 512 0.25
2.004034001 xfsaild/md0 456 xvdb W 62901520 8192 0.35
2.004042000 xfsaild/md0 456 xvdb W 63542016 4096 0.36
2.004204001 kworker/0:3 26040 xvdb W 41950344 65536 0.34
2.044352002 supervise 1950 xvda1 W 13192672 4096 0.65
2.044574000 supervise 1950 xvda1 W 13189072 4096 0.58
This includes the PID and comm (process name) that were on-CPU at the time of
issue (which usually means the process responsible).
The latency of the disk I/O, measured from the issue to the device to its
completion, is included as the last column.
This example output is from what should be an idle system, however, the
following is visible in iostat:
$ iostat -x 1
[...]
avg-cpu: %user %nice %system %iowait %steal %idle
0.12 0.00 0.12 0.00 0.00 99.75
Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s await svctm %util
xvda 0.00 0.00 0.00 4.00 0.00 16.00 0.00 0.00 0.00
xvdb 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
xvdc 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
md0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
There are 4 write IOPS.
The output of biosnoop identifies the reason: multiple supervise processes are
issuing writes to the xvda1 disk. I can now drill down on supervise using other
tools to understand its file system workload.