NAME¶

iosnoop - trace block I/O events as they occur. Uses Linux ftrace.

SYNOPSIS¶

iosnoop [-hQst] [-d device] [-i iotype] [-p pid] [-n name] [duration]

DESCRIPTION¶

iosnoop prints block device I/O events as they happen, with useful details such as PID, device, I/O type, block number, I/O size, and latency.

This traces disk I/O at the block device interface, using the block: tracepoints. This can help characterize the I/O requested for the storage devices and their resulting performance. I/O completions can also be studied event-by-event for debugging disk and controller I/O scheduling issues.

NOTE: Use of a duration buffers I/O, which reduces overheads, but this also introduces a limit to the number of I/O that will be captured. See the duration section in OPTIONS.

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

REQUIREMENTS¶

FTRACE CONFIG, and the tracepoints block:block_rq_insert, block:block_rq_issue, and block:block_rq_complete, which you may already have enabled and available on recent Linux kernels. And awk.

OPTIONS¶

-d device

Only show I/O issued by this device. (eg, "202,1"). This matches the DEV column in the iosnoop output, and is filtered in-kernel.

-i iotype

Only show I/O issued that matches this I/O type. This matches the TYPE column in the iosnoop output, and wildcards ("*") can be used at the beginning or end (only). Eg, "*R*" matches all reads. This is filtered in-kernel.

-p PID

Only show I/O issued by this PID. This filters in-kernel. Note that I/O may be issued indirectly; for example, as the result of a memory allocation, causing dirty buffers (maybe from another PID) to be written to storage.

With the -Q option, the identified PID is more accurate, however, LATms now includes queueing time (see the -Q option).

-n name

Only show I/O issued by processes with this name. Partial strings and regular expressions are allowed. This is a post-filter, so all I/O is traced and then filtered in user space. As with PID, this includes indirectly issued I/O, and -Q can be used to improve accuracy (see the -Q option).

-h

Print usage message.

-Q

Use block I/O queue insertion as the start tracepoint (block:block_rq_insert), instead of block I/O issue (block:block_rq_issue). This makes the following changes: COMM and PID are more likely to identify the origin process, as are -p PID and -n name; STARTs shows queue insert; and LATms shows I/O time including time spent on the block I/O queue.

-s

Include a column for the start time (issue time) of the I/O, in seconds. If the -Q option is used, this is the time the I/O is inserted on the block I/O queue.

-t

Include a column for the completion time of the I/O, in seconds.

duration

Set the duration of tracing, in seconds. Trace output will be buffered and printed at the end. This also reduces overheads by buffering in-kernel, instead of printing events as they occur.

The ftrace buffer has a fixed size per-CPU (see /sys/kernel/debug/tracing/buffer_size_kb). If you think events are missing, try increasing that size (the bufsize_kb setting in iosnoop). With the default setting (4 Mbytes), I'd expect this to happen around 50k I/O.

EXAMPLES¶

Default output, print I/O activity as it occurs:

# iosnoop

Buffer for 5 seconds (lower overhead) and write to a file:

# iosnoop 5 > outfile

Trace based on block I/O queue insertion, showing queueing time:

# iosnoop -Q

Trace reads only:

# iosnoop -i '*R*'

Trace I/O issued to device 202,1 only:

# iosnoop -d 202,1

Include I/O start and completion timestamps:

# iosnoop -ts

Include I/O queueing and completion timestamps:

# iosnop -Qts

Trace I/O issued when PID 181 was on-CPU only:

# iosnoop -p 181

Trace I/O queued when PID 181 was on-CPU (more accurate), and include queue time:

# iosnoop -Qp 181

FIELDS¶

COMM

Process name (command) for the PID that was on-CPU when the I/O was issued, or inserted if -Q is used. See PID. This column is truncated to 12 characters.

PID

Process ID which was on-CPU when the I/O was issued, or inserted if -Q is used. This will usually be the process directly requesting I/O, however, it may also include indirect I/O. For example, a memory allocation by this PID which causes dirty memory from another PID to be flushed to disk.

TYPE

Type of I/O. R=read, W=write, M=metadata, S=sync, A=readahead, F=flush or FUA (force unit access), D=discard, E=secure, N=null (not RWFD).

DEV

Storage device ID.

BLOCK

Disk block for the operation (location, relative to this device).

BYTES

Size of the I/O, in bytes.

LATms

Latency (time) for the I/O, in milliseconds.

OVERHEAD¶

By default, iosnoop works without buffering, printing I/O events as they happen (uses trace_pipe), context switching and consuming CPU to do so. This has a limit of about 10,000 IOPS (depending on your platform), at which point iosnoop will be consuming 1 CPU. The duration mode uses buffering, and can handle much higher IOPS rates, however, the buffer has a limit of about 50,000 I/O, after which events will be dropped. You can tune this with bufsize_kb, which is per-CPU. Also note that the "-n" option is currently post-filtered, so all events are traced.

The overhead may be acceptable in many situations. If it isn't, this tool can be reimplemented in C, or using a different tracer (eg, perf_events, SystemTap, ktap.)

SOURCE¶

This is from the perf-tools collection.

https://github.com/brendangregg/perf-tools

Also look under the examples directory for a text file containing example usage, output, and commentary for this tool.

OS¶

Linux

STABILITY¶

Unstable - in development.

AUTHOR¶

Brendan Gregg

SEE ALSO¶

iolatency(8), iostat(1), lsblk(8)