bcc-biosnoop - Man Page

biosnoop [-h] [-Q] [-d DISK] [-P]

This tools traces block device I/O (disk I/O), and prints a one-line summary for each I/O showing various details. These include the latency from the time of issue to the device to its completion, and the PID and process name from when the I/O was first created (which usually identifies the responsible process).

This uses in-kernel eBPF maps to cache process details (PID and comm) by I/O request, as well as a starting timestamp for calculating I/O latency.

This works by tracing various kernel blk_*() functions using dynamic tracing, and will need updating to match any changes to these functions.

This makes use of a Linux 4.4 feature (bpf_perf_event_output()); for kernels older than 4.4, see the version under tools/old, which uses an older mechanism

Since this uses BPF, only the root user can use this tool.

CONFIG_BPF and bcc.

-h

Print usage message.

-Q

Include a column showing the time spent queued in the OS.

-d DISK

Trace this disk only.

-P

Display block I/O pattern (sequential or random).

Trace all block device I/O and print a summary line per I/O:

# biosnoop

TIME(s)

Time of the I/O completion, in seconds since the first I/O was seen.

COMM

Cached process name, if present. This usually (but isn't guaranteed) to identify the responsible process for the I/O.

PID

Cached process ID, if present. This usually (but isn't guaranteed) to identify the responsible process for the I/O.

DISK

Disk device name.

T

Type of I/O: R = read, W = write. This is a simplification.

SECTOR

Device sector for the I/O.

BYTES

Size of the I/O, in bytes.

QUE(ms)

Time the I/O was queued in the OS before being issued to the device, in milliseconds.

LAT(ms)

Time for the I/O (latency) from the issue to the device, to its completion, in milliseconds.

Since block device I/O usually has a relatively low frequency (< 10,000/s), the overhead for this tool is expected to be negligible. For high IOPS storage systems, test and quantify before use.

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.

Linux

Unstable - in development.

Brendan Gregg, Rocky Xing

disksnoop(8), iostat(1)