While perusing the catonmat blog, I came across a reference to the pv utility. Pv allows you to monitor the amount of data that is written to a pipe, which is incredibly useful for monitoring how much data is transferred between pipe endpoints. The following example shows this super useful tool in action:
$ dd if=/dev/zero | pv > foo
522MB 0:00:06 [ 109MB/s] [ <=> ]
When pv is added to the pipeline, you get a continuous display of the amount of data that is being transferred between two pipe endpoints. I really dig this utility, and I am stoked that I found the catonmat website! Niiiiiiiiice!
[root@machine2 ~]# ssh machine1 “dd if=/dev/VolGroup00/domU2migrate”|pv -s 8G -petr|dd of=/dev/xen02vg/domU2migrate
0:00:30 [11.2MB/s] [====> ] 4% ETA :10:13
Want to rate limit the transfer so you don’t flood the pipe?
-L RATE, –rate-limit RATE
Limit the transfer to a maximum of RATE bytes per second. A suffix of “k”, “m”, “g”, or “t” can be added to denote kilobytes (*1024), megabytes, and so on.
-B BYTES, –buffer-size BYTES
Use a transfer buffer size of BYTES bytes. A suffix of “k”, “m”, “g”, or “t” can be added to denote kilobytes (*1024), megabytes, and so on. The default buffer size is the block size of the input file’s filesystem multiplied by 32 (512kb max), or 400kb if the block size cannot be determined.
Already have a transfer in progress and want to rate limit it without restarting?
-R PID, –remote PID
If PID is an instance of pv that is already running, -R PID will cause that instance to act as though it had been given this instance’s command line instead. For example, if pv -L 123k is running with process ID 9876, then running pv -R 9876 -L 321k will cause it to start using a rate limit of 321k instead of 123k. Note that some options cannot be changed while running, such as -c, -l, and -f.