swap space recommendation on linux

[skt_skt]

# cat /etc/redhat-release
Red Hat Linux Advanced Server release 2.1AS (Pensacola)

# uname -a
Linux hostname 2.4.9-e.71smp #1 SMP Thu Nov 23 11:51:48 EST 2006 i686 unknown

i have 4GB RAM and 4GB swap currently. often the swap goes beyond 85%.

What would be the ideal RAM/SWAP ratio?what are the recommendations when we add the swap online?I would like to add additinal swap space from an existing disk /dev/sdb. I dont have LVM.My plan is to create one 2GB partion on /dev/sdb


# swapon -s
Filename Type Size Used Priority
/dev/sda2 partition 2097136 1548124 -1
/dev/sdb1 partition 2097136 1268328 -2

[Ivan Ferreira · Accepted solution]

According to the installation guide, swap should be:

Swap should equal 2x physical RAM for up to 2 GB of physical RAM, and then 1x physical RAM for any amount above 2 GB, but never less than 32 MB.

Using this formula, a system with 2 GB of physical RAM would have 4 GB of swap, while one with 3 GB of physical RAM would have 5 GB of swap. Creating a large swap space partition can be especially helpful if you plan to upgrade your RAM at a later time.

[Stuart Browne]

Your swap utilization is over 85% ?!?

I'd be suggesting you invest in more memory!

[dirk dierickx]

there is no ideal formula. it depends on the type of app you're running and the load caused by that app.

all our linux servers share the same install, but you'll find different swap utilization on all of them.

[~sesh]

Ideally for a system with 4 GB of memory swap file utilization should be minimum.

Normally swap file is set to 1.5 - 2.0x the physical memory. However, as the memory increases, the rule need not be adhered to.

There is also a parameter called swappiness (/proc/sys/vm/swappiness), if you are sure that the physical memory is sufficient for the operations and that swap file need not be used much you can reduce the swappiness.

It goes from 0 - 100, higher the number, greater the tendency to use the swap file. You can read more about swappiness at:

http://kerneltrap.org/node/3000

[Jeroen Peereboom]

If your system is swapping, either investigate why it needs so much memory or just buy more memory. IMHO a server should use minimal swap space.

JP

[skt_skt]

below is the information i see ..As per that i cannot add another swap partition if the same disk is already in use as FS or a swap partition.

"How do I add a swap partition to my system in Red Hat Enterprise Linux? Resolution:To add a swap partition (assuming /dev/hdb2 is the swap partition you want to add): The hard drive can not be in use (partitions can not be mounted, and swap space can not be enabled). The partition table should not be modified while in use because the kernel may not properly recognize the changes. Data could be overwitten by writing to the wrong partition because the partition table and partitions mounted do not match. The easiest way to achieve this is to boot your system in rescue mode. When prompted to mount the file system, select Skip.

Alternately, if the drive does not contain any partitions in use, you can unmount them and turn off all the swap space on the hard drive with the swapoff command."

is there anyway i can achieve adding another swap partition from same disk w/o taking down time?

[Stuart Browne]

If there is available space on the disk, then you should be able to.

If you've got no available space (unpartitioned space), but available space in a filesystem, then you can create 'swap files', and use them as swap space instead to get you past a temporary issue.

The details on how to do that are in the 'mkswap' man page.

[skt_skt]

But the recommendation/warning posted in "quote" (from parted)do NOT allow me to go ahead and try this. Also this is a PROD box and dont want to give a chance

i have enough space on this particular disk. But there is an existing swap partition on the same disk.

I started with fdisk ; created a partion with id 82(Linux-swap)

But swapon was failing .

#swapon /dev/sdb2
swapon: /dev/sdb2: No such device or address

Then i started with parted. So what i posted in quote earlier are the recommendations from parted.

"mkpart PART-TYPE [FS-TYPE] START END make a partition"

[skt_skt]

the ansers to "swap space recommendation on linux" were pretty help full.

but adding swap online from an existing disk(swap enabled with one partion) is not possible with 2.1 kernel.

other choice to add file swap whihc i dont prefer for this scenario.

[sandeep mathur]

firstly, swap size must be double the size of RAM and then while making partitions in mount you have to specify the size of ram and swap in such a way that swap size must be double the size of physical memory.