iosnoop(1m)

NAME
iosnoop - snoop I/O events as they occur. Uses DTrace.

SYNOPSIS
iosnoop [-a|-A|-Deghinostv] [-d device] [-f filename] [-m mount_point] [-n name] [-p PID]

DESCRIPTION
iosnoop prints I/O events as they happen, with useful details such as UID, PID, block number, size, filename, etc. This is useful to determine the process responsible for using the disks, as well as details on what activity the process is requesting. Behaviour such as random or sequential I/O can be observed by reading the block numbers. Since this uses DTrace, only users with root privileges can run this command.

OPTIONS

-a  print all data

-A  dump all data, space delimited

-D  print time delta, us (elapsed)

-e  print device name

-i  print device instance

-N  print major and minor numbers

-o  print disk delta time, us

-s  print start time, us

-t  print completion time, us

-v  print completion time, string

-d device
    instance name to snoop (eg, dad0)

-f filename
    full pathname of file to snoop

-m mount_point
    mountpoint for filesystem to snoop

-n name
    process name

-p PID
    process ID

EXAMPLES

Default output, print I/O activity as it occurs,
    # iosnoop

Print human readable timestamps,
    # iosnoop -v

Print major and minor numbers,
    # iosnoop -N

Snoop events on the root filesystem only,
    # iosnoop -m /

FIELDS

UID User ID

PID Process ID

PPID
    Parent Process ID

COMM
    command name for the process

ARGS
    argument listing for the process

SIZE
    size of the operation, bytes

BLOCK
    disk block for the operation (location. relative to this filesystem. more useful with the -N option to print major and minor numbers)

STIME
    timestamp for the disk request, us

TIME
    timestamp for the disk completion, us

DELTA
    elapsed time from request to completion, us (this is the elapsed time from the disk request (strategy) to the disk completion (iodone))

DTIME
    time for disk to complete request, us (this is the time for the disk to complete that event since it's last event (time between iodones), or, the time to the strategy if the disk had been idle)

STRTIME
    timestamp for the disk completion, string

DEVICE
    device name

INS device instance number

D   direction, Read or Write

MOUNT
    mount point

FILE
    filename (basename) for I/O operation

NOTES
When filtering on PID or process name, be aware that poor disk event times may be due to events that have been filtered away, for example another process that may be seeking the disk heads elsewhere.

DOCUMENTATION
See the DTraceToolkit for further documentation under the Docs directory. The DTraceToolkit docs may include full worked examples with verbose descriptions explaining the output.

EXIT
iosnoop will run forever until Ctrl-C is hit.

AUTHOR
Brendan Gregg [Sydney, Australia]

SEE ALSO
iotop(1M), dtrace(1M)