http://www.cyberciti.biz/tips/linux-swap-space.html
Linux and other Unix-like operating systems use the term "swap" to describe both the act of moving memory pages between RAM and disk, and the region of a disk the pages are stored on. It is common to use a whole partition of a hard disk for swapping. However, with the 2.6 Linux kernel, swap files are just as fast as swap partitions. Now, many admins (both Windows and Linux/UNIX) follow an old rule of thumb that your swap partition should be twice the size of your main system RAM. Let us say I've 32GB RAM, should I set swap space to 64 GB? Is 64 GB of swap space really required? How big should your Linux / UNIX swap space be?
Old dumb memory managers
I think the '2x swap space' rule came from Old Solaris and Windows admins. Also, earlier memory mangers were very badly designed. There were not very smart. Today, we have very smart and intelligent memory manager for both Linux and UNIX.
Nonsense rule: Twice the size of your main system RAM for Servers
According to OpenBSD FAQ:
Many people follow an old rule of thumb that your swap partition should be twice the size of your main system RAM. This rule is nonsense. On a modern system, that's a LOT of swap, most people prefer that their systems never swap. You don't want your system to ever run out of RAM+swap, but you usually would rather have enough RAM in the system so it doesn't need to swap.
Select right size for your setup
Here is my rule for normal server (Web / Mail etc):
Swap space == Equal RAM size (if RAM < 2GB)
Swap space == 2GB size (if RAM > 2GB)
Many Oracle experts recommend something as follows for heavy duty Oracle server with fast storage such as RAID 10:
Swap space == Equal RAM size (if RAM < 8GB)
Swap space == 0.50 times the size of RAM (if RAM > 8GB)
However, newer servers can have
Red Hat Recommendation
Red hat recommends setting as follows for RHEL 5:
The reality is the amount of swap space a system needs is not really a function of the amount of RAM it has but rather the memory workload that is running on that system. A Red Hat Enterprise Linux 5 system will run just fine with no swap space at all as long as the sum of anonymous memory and system V shared memory is less than about 3/4 the amount of RAM. In this case the system will simply lock the anonymous and system V shared memory into RAM and use the remaining RAM for caching file system data so when memory is exhausted the kernel only reclaims pagecache memory.
Considering that 1) At installation time when configuring the swap space there is no easy way to predetermine the memory a workload will require, and 2) The more RAM a system has the less swap space it typically needs, a better swap space
Systems with 4GB of ram or less require a minimum of 2GB of swap space
Systems with 4GB to 16GB of ram require a minimum of 4GB of swap space
Systems with 16GB to 64GB of ram require a minimum of 8GB of swap space
Systems with 64GB to 256GB of ram require a minimum of 16GB of swap space
Swap will just keep running servers...
Swap space will just keep operation running for a while on heavy duty servers by swapping process. You can always find out swap space utilization using any one of the following command:
cat /proc/swaps
swapon -s
free -m
top
See how to find out disk I/O and related information under Linux. In the end, you need to add more RAM, adjust software (like controlling Apache workers or using lighttpd web server to save RAM) or use some sort of load balancing.
Also, refer Linux kernel documentation for /proc/sys/vm/swappiness. With this you can fine tune swap space.
A note about Desktop and Laptop
If you are going to suspend to disk, then you need swap space more than actual RAM. For example, my laptop has 1GB RAM and swap is setup to 2GB. This only applies to Laptop or desktop but not to servers.
Kernel hackers need more swap space
If you are a kernel hacker (debugging and fixing kernel issues) and generating core dumps, you need twice the RAM swap space.
Conclusion
If Linux kernel is going to use more than 2GB swap space at a time, all users will feel the heat. Either, you get more RAM (recommend) and move to faster storage to improve disk I/O. There are no rules, each setup and configuration is unique. Adjust values as per your requirements. Select amount of swap that is right for you.
