Side Channel

Back in the early days of Linux, the rule of thumb for sizing your swap partition is "twice the amount of RAM". But does that still apply to modern hardware where we regularly have 16 GB or more?

4 schools of thought:

• $ram * 2 (I call it the "jurassic" method)
• $ram > 2048 ? $ram + 2048 : $ram * 2 (aka "redhat", see below)
• $ram <= 2 GB ? 1.5 * $ram : ($ram <= 8 GB ? $ram : 0.75 * $ram) (aka "oracle", see below)
• 4GB (I call it "angch's")

Last school of thought is mine, and the rationale behind it includes:

• Swapping is slow. If you're filling up 4GB of swap, that going to take quite some time. Swapping your leaky Firefox to and from disk is going to make you very annoyed.
• If you're already swapping more than 1 gig of RAM, something is very, very wrong with your system. Adding more swap is just going to delay the inevitable OOM killer triggering.
• 4GB ought to be enough for everyone.

Doesn't apply to netbooks, of course. And just make sure you have enough to hibernate your notebooks.

• Red Hat RHEL4 method: http://www.redhat.com/docs/manuals/enterprise/RHEL-4-Manual/sysadmin-gui...
• Oracle: http://www.puschitz.com/TuningLinuxForOracle.shtml#SizingSwapSpace
• Oracle 10gr2: http://download-east.oracle.com/docs/cd/B19306_01/install.102/b15660/pre...
• http://dsumsky.blogspot.com/2008/12/quickly-linux-swap-space-sizing.html

---

Handy table for you to size your swap space, with swap space in MB for fdisk friendliness.

RAM size	Swap size, Jurassic	Swap size, RedHat	Swap size, Oracle	Swap size, angch
256 MB	512 MB	512 MB	(n/a)	4096 MB (you're gonna upgrade your RAM soon, right?)
512 MB	1024 MB	1024 MB	(n/a)	4096 MB
1024 MB (1GB)	2048 MB	2048 MB	1536 MB	4096 MB
2GB	4096 MB	4096 MB	3072 MB	4096 MB
4GB	8192 MB	6144 MB	4096 MB	4096 MB
8GB	16384 MB	10240 MB	8192 MB	4096 MB
16GB	32768 MB	18432 MB	12288 MB	4096 MB
32GB	65536 MB	34816 MB	24576 MB	4096 MB

4 GB seems to be the Mach barrier of sorts for modern operating systems:

• if you have >= 4GB RAM, use a 64bit OS.
• if you have >= 4GB RAM, 4GB swap should be sufficient.

P.S. I wimped out on my own recommendation and allocated 18GB swap for the new 16GB machines (redhat method). Hypocrite.