Two swaps on a single disk

[d.yoman]

What is the effect of having two swaps on a single disk? How much performance degradation may take place?

[Jeff Schussele]

Hi,

Well if you're actually paging out then it wouldn't make any difference.
Performance would already be in the crapper.
It would be as if it was all one big swap space on that disk.
You never *ever* want to page out - disks are 1000s of time slower than RAM.

My 2 cents,
Jeff

[Pete Randall]

If you should start swapping, there is going to be massive head contention but once you've started swapping, your performance is going to be so bad that you'll never notice it.


Pete

[Kent Ostby]

Since a good chunk of setting up swap is "just in case we have to swap" (i.e. the "reserve" line), it probably isn't going to make a big difference.

[James R. Ferguson]

Hi:

You never want to swap.

If you haven't already, enable "pseudoswap" and you may not even need a secondary swap.

Enabling the kernel parameter 'swapmem_on' allows 75% of your physical memory to be counted by the kernel for the purposes of process swap reservation. This can greatly help avoid "not enough memory" errors during process startup.

Regards!

...JRF...

[d.yoman]

Lets say i have 8GB of existing swap and i want to have additional 4 GB of swap. So where would you recommend me to put the additional swap of 4GB? 1)on the same disk as of my original 8GB but using a different lvol, or 2) on another disk?

[A. Clay Stephenson]

From a pure performance standpoint, you should put the two swap areas on different disks -- at the same priority. This will interleave the swap between the two disks.

If you choose to put them on the same physical disk then they should be set to different priorities so that the lower priority is used completely before the next priority is used.

Now having said all this, the performance impact is so great from swapping itself that the actual swap layout is insignificant. You are rearranging the deck chairs on the Titanic.