Swap in systems with large amount Memory.

[Rusty Sapper]

Every HP Document I can find says the same thing.

"Typically, unless the amount of physical memory on your system is extremely large, the minimum amount of swap space should equal the amount of physical memory on the system"

So, what is the HP advice for systems WITH large amounts of Swap space? Say, a Server with 32GB of MEM. I know how to determine the swap requirements for the Apps that I'm running, but I would like to get HP's take on it.

Any thoughts?

TIA

-Rusty

[Eugeny Brychkov]

As I remember recommended size of swap equals twice size of memory installed. Why >=equal? My guess is that usually swap space is configured as dump space and if machine will crashdump then swap area should be able to contain whole memory image at crash to allow it to be saved to /var/adm/crash and have complete image for further troubleshooting by HP engineers. This is my opinion
Eugeny

[A. Clay Stephenson]

32GB's of swap qualifies as lots of memory. In that case, I would configure only about 512MB of primary swap (you must have some) and enable pseudoswap and that would be it. I assume with that much memory you will never swap.

The old rules about 2x-3x swap space very seledom apply anymore. There are a few exceptions. Some software requires large amounts of swap (the install actually checks) but as soon as the install is finished, you can remove the swap.

Whatever swap space you do use, I strongly urge you to mirror it so that the system will always be up. I always configure separate dump space (unmirrored).

Probably as close to an official rule these days that you will get is: 1) Configure a small amount of primary swap, enable pseduswap, and monitor the system. Swap is so easy to add later that it really no worth worrying about especially if you go ahead and adjust maxswapchunks upward to accodate any possibly needed additional swap.

[eran maor]

Hi

i will lister to harry advise , you dont need to define 32 of g.b of swap ,
you need also to check the status of the memory on your system , i think that 1-2 g.b is more then anougth to define , but you can lister to harry and define 500 m.b and if you will see at glance or with swapinfo that you are using the swap increase the size .

the guide line of HP recommend 1-2 time of swap of memory but there are talking about reguler system with 1-2 g.b of ram , over there it is commom to follow this rull

[Steven E. Protter]

I have heard from numerous HP people with solid reps that above 16G the twice phyiscal memory rule does not apply. Less will do.

There are people that disagree, but that is to be expected.

Steve

[fg_1]

Rusty

I concurr totally with the posting from james,
but configure about 1GB of primary swap, then
utilize the pseudoswap.

On a similar subject I would also configure a seperate filesystem with enough space to hold
a crash dump. Typically we configure 105% of physical memory but the largest memory config we have here is 12GB on a v2600.

GL.

Frank G.

[fg_1]

Sorry, posting was from A.CLAY, not james.
my bag.

[CITEC HP TEAM]

This is good advice and along the lines I have been thinking. We will soon be setting up a system with 60 gb of memory.

I would like to add pseudo swap into the picture for your consideration. My understanding is that up to 75% of physical memory is allocated as pseudo swap, although I have seen a figure of 7/8 as well.

My understanding of this is that you would need to allocate swape for the rest in order to use all of physical memory. ie 25% of physical memory. Is this correct?

[Sridhar Bhaskarla]

CITEC HP TEAM

You are right. However, I try to be bit cautious and usually configure atleast 50% of memory as device swap in addition to the pseudo swap. And it is not necessarily the primary swap.

One word of caution with 60GB of memory though. If this is a production system, spare a disk for dump. You may need around 10 GB to start with. Once the system is fully functional and under heavy load run 'crashconf' and determine your dump requirements and add your dump logical volume accordingly.

-Sri

[Steven E. Protter]

On the low side, I've tried doing OS installs with the intitial swap/dump area less than half of RAM. The Ignite interface complained that the minimum amount of swap/dump needed to receive a dump is 50% of physical memory.

I think it reasonable to believe in this case that that is the low end you can set swap/dump. You may be able to override it, but only at the possible cost of a system that can't dump.

SEP