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]