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Ramin Firoozye's (occasional) Public Whisperings

Slowing down Time Machine

Posted by: Ramin on November 8, 2008

Time Machine is great, especially if hooked up to a network storage device like Drobo so you can just have it run in the background. In my case the Drobo is connected to a Mac Mini acting as a network file server (yes, I know it’s not officially supported, but it works fine under Leopard).

However, lately the backups have been taking a lot of system and network resources, rendering the machine (in this case my development laptop) practically unusable while they run. But since Time Machine runs in the background, it’s OK to lower the backup daemon’s priority and let it run a little slower.

On Mac OS X Leopard, this is the command that does the trick. From the terminal:

sudo renice +5 -p `ps -axc | grep backupd | awk '{ print \$1 }'`

Here’s what’s going on:

  • sudo – This runs the command as the root. You will need to enter the administrator password.
  • renice – This is the standard Unix command for changing the priority of a running application.
  • +5: Process priorities under BSD Unix-based systems typicall run from -20 to +20, with +20 being lowest (i.e. running slowest) to -20 being maximum (yes, I know it’s unintuitively backward, but there’s an old historical reason for it). What we’re doing is bumping Time Machine daemon’s nice priority up by 5 (or any number you want) to let it run slower.
  • -p pid – This is the process id of the process you want to adjust. Since this changes every time Time Machine runs, we have to have a way to find it dynamically at runtime — which is where the rest of the line enclosed in ` back-quotes come in. On most Unix shells, items enclosed in back-quotes get executed and the result returned back to the command line. So we’re going to look up the process ID of the current Time Machine server process and return it here.
  • ps -axc – The ps command returns a long list of all running processes on the system. We need to filter out the one we want, which we do by piping the output into a grep filter next…
  • grep backupd – We’re taking all the output from the ps command and only keeping those lines that contain the string backupd — which happens to be the name of the Time Machine server. So we end up with a single line of ps output that looks something like this:

    19041 ?? 1:07.48 backupd

    But what we need is the process ID to pass back up to the renice command. In this case, it’s the first number on the line. We need a way to extract only that, which is where awk — the amazing text processing Swiss-Army knife — comes in…

  • awk “{print \$1 }” – By default, awk splits its input into chunks based on whitespace. We’re simply asking that the first item be returned. Any time you do this, you should apologize to awk for so massively under-utilizing what it can do. It’s like driving your Formula-1 car down to the grocery store to buy milk. In this case all we’re doing is asking it to split up some text and return one item to us, something that awk can do practically in its sleep.

When you run this, the sudo part of the command will ask you for your admin password, then proceed to do its thing. Put it all together and you’ve got yourself a simple way to slow down Time Machine so it’s not such a CPU hog.

If you’re enterprising, you can put the whole thing into a shell function and run it over and over. The following code goes inside your .bash_profile file.

function tmslow {
    echo "Reducing Time Machine priority..."
    sudo renice +5 -p `ps -axc | grep backupd | awk '{ print \$1 }'`

Start a new terminal session (to make sure the shell function is loaded) then invoke tmslow and enter your admin password. It prints out a little message reminding you what it’s about to do.

Remember, the renice command doesn’t stick, so every time you reboot or a new Time Machine session starts, the process goes back to normal priority. There are ways to automate the priority lowering scheme or even make it permanent, but I don’t recommend doing that. Sometimes, you may want backups to run full-speed. Unix makes it trivial to do what you want by stringing together some built-in commands.