Two Swap Space on diff Disks

[Laurie_2]

OK could someone please
educate me here.

We do not have disk mirroring
(I know very bad), yet
we have our swap space set
up as follows:

/dev/vg00/lvol2 2 672 now/on boot
/dev/vg01/lvol1 2 Device 672 now/on boot

What I don't understand is
the Current priority. vg00 is priority 2 and vg01 is priority 1.

Does this mean we are only using (if needed) 672MB of swap and if one disk crashes
we get the other disk of swap?

Or does this mean we have two
disks of 672MB of swap space giving us a grant total of 1344MB of swap?

We have 2GB of RAm and someone told us (techie vendor) that we should have
3 x the swap space of memory.
Then they said we should not
have 8 disks on one controller, then another vendor said SCSI Fast/Wide can
support 15 disks on problem.

Boy the more I learn about unix, the more different opinions....

Anyway, please education me on
swap space here. Either we have 672 or double?

We don't have disk mirroring
but I am working on setting that up.
TIA,
Laurie

[Sandip Ghosh · Accepted solution]

Hi,

First : Both the swap space should have same priority. By default, during installation, it used to set as priority 1.

Second : I think they had given Swap=3xmemory
instead of memory = 3xswap. Because according to older method (HP-UX 9.X) we use to keep the swapspace = 3xmemory. Now on HP-UX 11.x we do not need that much of Swap area. It depends on the amount of memory.

Now according to your setting , if the First swap area (vg01) get exhausted then the second swap area (vg00) will come into effect. And you can use max 672*2 swap area.

Sandip

[A. Clay Stephenson]

Hi Laurie:

The old rule about 3x swap for RAM very seldom applies. However, in your case, unless you have at least as much swap space as you have RAM or have pseudo-swap on, you are not going to be able to use all of your virtual memory. Run vmstat and get some baseline values; if you see very low pageout rates (po - the only significant value in vmstat) then you know that you are really swapping.

Because your swap devices are on different devices, you should set the priorities to 1. Just make the change in /etc/fstab and the next time you reboot, you are done. Having said that, if you find that you are not swapping then it doesn't matter.

[Patrick Wallek]

To answer you disk / scsi question; FWD scsi CAN support up to 15 disk drives in the scsi chain (0-15, minus 1 id for the SCSI controller), but that is not to say that you SHOULD put that many devices on one chain.

If you have an application that is very I/O intensive, then you are going to cause some disk bottlenecks if you have 15 drives on the same chain. Even 8 could be a bit high. For the best throughput you should always spread you drives over as many controllers as possible. If you use striping to spread out the I/O that's even better.