iosnoop - trace block I/O events as they occur. Uses Linux
ftrace.
iosnoop [-hQst] [-d device] [-i iotype] [-p pid] [-n name]
[duration]
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.
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.
- -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.
- 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.
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.)
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.
Unstable - in development.
iolatency(8), iostat(1), lsblk(8)