I have been able to move Apple's virtual memory swapfiles to a dedicated partition on my hard drive up until now.
The technique I have been using is described in a thread on forums.macosxhints.com.
However, with
the developer preview of Snow Leopard, this method no longer works. Does anyone know how it could be done with
the new OS?
Update: I have marked dblu's answer as accepted even though it didn't quite work because he gave excellent, detailed instructions and because his suggestion to use plutil ultimately pointed me in
the right direction.
The complete, working solution is posted here in
the question because I don't have enough reputation to edit
the accepted answer.
Complete solution:
1. Open Terminal and make a backup copy of Apple's default dynamic_pager.plist:
$ cd /System/Library/LaunchDaemons
$ sudo cp com.apple.dynamic_pager.plist{,_bak}
2. Convert
the plist from binary to plain XML:
$ sudo plutil -convert xml1 com.apple.dynamic_pager.plist
3. Open
the converted plist with your text editor of choice. (I use pico, see dblu's answer for an example using vim):
$ sudo pico -w com.apple.dynamic_pager.plist
It should look as follows:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs$
<plist version="1.0">
<dict>
<key>EnableTransactions</key>
<true/>
<key>HopefullyExitsLast</key>
<true/>
<key>Label</key>
<string>com.apple.dynamic_pager</string>
<key>OnDemand</key>
<false/>
<key>ProgramArguments</key>
<array>
<string>/sbin/dynamic_pager</string>
<string>-F</string>
<string>/private/var/vm/swapfile</string>
</array>
</dict>
</plist>
4. Change
the ProgramArguments array (lines 13 through 18) so that it launches an intermediate shell script instead of launching dynamic_pager directly. See note #1 for details on why this is necessary.
<key>ProgramArguments</key>
<array>
<string>/sbin/dynamic_pager_init</string>
</array>
5.
Save the plist, and return to
the terminal prompt. Using pico,
the commands would be:
<ctrl+o> to
save the file
<enter> to accept
the same filename (com.apple.dynamic_pager.plist)
<ctrl+x> to exit
6. Convert
the modified plist back to binary:
$ sudo plutil -convert binary1 com.apple.dynamic_pager.plist
7. Create
the intermediate shell script:
$ cd /sbin
$ sudo pico -w dynamic_pager_init
The script should look as follows (my partition is called 'Swap', and I chose to put
the swapfiles in a hidden directory on that partition, called '.vm' be sure that
the directory you specify actually exists):
Update: This version of
the script makes use of wait4path as suggested by ZILjr:
#!/bin/bash
#launch Apple's dynamic_pager only when
the swap volume is mounted
echo "Waiting for Swap volume to mount";
wait4path /Volumes/Swap;
echo "Launching dynamic pager on volume Swap";
/sbin/dynamic_pager -F /Volumes/Swap/.vm/swapfile;
8.
Save and close dynamic_pager_init (same commands as step 5)
9. Modify permissions and ownership for dynamic_pager_init:
$ sudo chmod a+x-w /sbin/dynamic_pager_init
$ sudo chown root:wheel /sbin/dynamic_pager_init
10. Verify
the permissions on dynamic_pager_init:
$ ls -l dynamic_pager_init
-r-xr-xr-x 1 root wheel 6 18 Sep 15:11 dynamic_pager_init
11. Restart your Mac. If you run into trouble, switch to verbose startup mode by holding down Command-v immediately after
the startup chime. This will let you see all of
the startup messages that appear during startup. If you run into even worse trouble (i.e. you never see
the login screen), hold down Command-s instead. This will boot
the computer in single-user mode (no graphical UI, just a command prompt) and allow you to restore
the backup copy of com.apple.dynamic_pager.plist that you made in step 1.
12. Once
the computer boots, fire up Terminal and verify that
the swap files have actually been moved:
$ cd /Volumes/Swap/.vm
$ ls -l
You should see something like this:
-rw------- 1 someUser staff 67108864 18 Sep 12:02 swapfile0
13. Delete
the old swapfiles:
$ cd /private/var/vm
$ sudo rm swapfile*
14. Profit!
Note 1
Simply modifying
the arguments to dynamic_pager in
the plist does not always work, and when it fails, it does so in a spectacularly silent way.
The problem stems from
the fact that dynamic_pager is launched very early in
the startup process. If your swap partition has not yet been mounted when dynamic_pager is first loaded (in my experience, this happens 99% of
the time), then
the system will fake its way through. It will create a symbolic link in your /Volumes directory which has
the same name as your swap partition, but points back to
the default swapfile location (/private/var/vm). Then, when your actual swap partition mounts, it will be given
the name Swap 1 (or YourDriveName 1). You can see
the problem by opening up Terminal and listing
the contents of your /Volumes directory:
$ cd /Volumes
$ ls -l
You will see something like this:
drwxrwxrwx 11 yourUser staff 442 16 Sep 12:13 Swap -> private/var/vm
drwxrwxrwx 14 yourUser staff 5 16 Sep 12:13 Swap 1
lrwxr-xr-x 1 root admin 1 17 Sep 12:01 System -> /
Note that this failure can be very hard to spot. If you were to check for
the swapfiles as I show in step 12, you would still see them!
The symbolic link would make it seem as though your swapfiles had been moved, even though they were actually being stored in
the default location.
Note 2
I was originally unable to get this to work in Snow Leopard because com.apple.dynamic_pager.plist was stored in binary format. I made a copy of
the original file and opened it with Apple's Property List Editor (available with Xcode) in order to make changes, but this process added some extended attributes to
the plist file which caused
the system to ignore it and just use
the defaults. As dblu pointed out, using plutil to convert
the file to plain XML works like a charm.
Note 3
You can check
the Console application to see any messages that dynamic_pager_init echos to
the screen. If you see
the following lines repeated over and over again, there is a problem with
the setup. I ran into these messages because I forgot to create
the '.vm' directory that I specified in dynamic_pager_init.
com.apple.launchd[1] (com.apple.dynamic_pager[176]) Exited with exit code: 1
com.apple.launchd[1] (com.apple.dynamic_pager) Throttling respawn: Will start in 10 seconds
When everything is working properly, you may see
the above message a couple of times, but you should also see
the following message, and then no more of
the "Throttling respawn" messages afterwards.
com.apple.dynamic_pager[???] Launching dynamic pager on volume Swap
This means that
the script did have to wait for
the partition to load, but in
the end it was successful.
