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)