swap using virtual memory instead of real memory

[Eric Hess]

when starting a instance on Informix the system is using swap space before it uses real memory. I am running 11.0. Has anyone encountered this before.
#swapinfo

dev 1048576 0 1048576 0% 0 - 1 /dev/vg00/lvol2
dev 1048576 190092 858484 18% 0 - 0 /dev/vg00/lvol9
dev 1048576 189164 859412 18% 0 - 0 /dev/vg00/lvol10
dev 1048576 188436 860140 18% 0 - 0 /dev/vg00/lvol11
dev 1048576 189456 859120 18% 0 - 0 /dev/vg00/lvol12
dev 1048576 162988 885588 16% 0 - 0 /dev/vg00/lvol13
dev 1048576 163404 885172 16% 0 - 0 /dev/vg00/lvol14
dev 1048576 162984 885592 16% 0 - 0 /dev/vg00/lvol15
dev 1048576 163084 885492 16% 0 - 0 /dev/vg00/lvol16
dev 1048576 163188 885388 16% 0 - 0 /dev/vg00/lvol17
dev 1048576 163420 885156 16% 0 - 0 /dev/vg00/lvol18
reserve - 3026888 -3026888
memory 2973796 228072 2745724 8%

[S.K. Chan]

The 8% represent your pseudo swap usage (ie reserved eveytime a process starts up). It's perfectly normal and it does not affect the perfromance of your server. Since your device swap "lvol2" had its priority set to "1" and you still have plenty of other device swap available (those at priority "0"), you would not see it being used until the rest of the swap space in priority "0" are used up.

[Sridhar Bhaskarla · Accepted solution]

Hi Eric,

Pseudo-swap is only to mimic the kernel that is has more swap available so that more number of processes can be brought up than that are limited by the device swap area. It is purely for reservation purposes and has nothing to do with memory being used for paging.

So, I am not worried for memory being shown as used for swap.

But your configuration looks worrysome to me. You have logical volumes 9-18 under vg00 configured with equal priority. This is like striping a single swap across multiple logical volumes with a stripe size equal to swchunk parameter. If these lvols are not distributed across different disk with one each, then your performance will be degraded heavily as the root disk has to work very hard in aligning the spindle on multiple locations each time a page-out happens along with it's normal business. Since you are already doing paging, I suggest you configure another disk with only one logical volume and make it a swap.

-Sri