Swap on external array...Opinions?

[Tom Smith_9]

I am interested in getting some opinions regarding adding additional swap space to an HP-UX system on an external disk array. I have long been of the opinion that it is unwise to use an external array for adding additional swap, but given the speed and reliability of modern disk arrays I am beginning to question that.

I have several HP 9000's running 11i, that are connected to an XP1024 via dual channel 2 GB FC HBA's. The OS and swap is currently installed on each on local SCSI disks. One system in particular, was not setup to use MirrorDisk, and at the moment I don't have the room to set it up without freeing up some disk space. The second disk that was meant to be used for mirroring was instead used to add 10 GB of additional swap, I have considered moving the 10 GB swap file to my XP array to free up the space I need for mirroring.

Thoughts?

Thanks,
-Tom

[Ashwani Kashyap · Accepted solution]

We have plenty of our swps on EMC arrays with no problems .

As long as all your extents are contiguous on that LUN , it shouldn't be a problem .

[A. Clay Stephenson]

I hate swapping so I never allow it to happen. It sounds as though your primary swap (which must be on the boot disk) is not mirrored. That would scare me because it means that your current system is very vulnerable --- not to mention that your other vg00 LVOL's are not mirrored.

You must have some primary swap space (as little as 512MB-1GB will be fine) and that must be on the boot disk. The remainder of your swap space can go on other HIGHLY AVAILABLE devices (that means mirrored disks or an array).

I tend to run systems with lots of memory with about 0.25 x RAM as swapspace with swapmem_on=1. I then monitor swap usage and if any more is needed I add it or buy more memory. The old rules about 2x-3x RAM for swapspace (4-6x when mirrored) very seldom apply these days. Finally, my rule, despite it being the default, is that swap space has absolutely nothing to do with dump space.

[Sanjay_6]

Hi tom,

you can use external arrays / disks / luns to add secondary swap. They can be non-contiguous as long as you don't use it as dump space. Under that condition it has to be contiguous.

Hope this helps.

Regds

[Steven E. Protter]

If you can boot off of it, you can swap to it. Even if you can't boot off of it, you may be able to swap on it.

Though if you swap on the external array and the fiber channel card goes down(that has happened to us) your machine stops.

This doomsday scenario can be mitigated by pvlinks and or mirroring.

Still, I don't like the idea. Maybe I'm just getting old.

Read A. Clay's post and take it to heart. I'm an A. Clay disciple with regards to swap.

SEP

[Tom Smith_9]

Thanks for the replies everyone. I am glad I posed the question.

Clay, you have described my situation pretty well. The server in question has 16 GB of RAM, I have a 6 GB primary swap file (lvol2) on the first internal disk (Along with the OS). The second internal disk was originally to be used for mirroring, but I was forced to add a 10 GB secondary swap file there instead. This gives me a 1x RAM swap ratio; this was a compromise with my primary customers, Oracle DBA's who asked for 2-3x RAM.

And, while this isn't a production system, I am nervous about not having any mirroring.

Your practice of not swapping sounds very interesting, I may try to setup a test server to see how it works in our environment.

Based on your opinions, I am definitely going to move the secondary swap to my array and setup mirroring on my second internal disk.

Thanks,
-Tom