Alan Schilla wrote:
Unless I missed it, no one has spoke to the pros/cons of multiple v-disk swap. I was discussing a number of things with Velocity software just yesterday and the recommendation given was to tune virtual storage down to the point swapping just begins and then set up 10 1 meg vdisk swap spaces rather than 1 10 meg. This provides a normal working set size and then will only allocate swap space that needs to be used. Would anyone more familar with this model care to elaborate? Al Schilla State of Minnesota
Most certainly! This is because Linux tends to pick new slots on the swap disk rather than re-use the ones that were freed up. Most of the slots used before will never be read by Linux again, but VM is not aware of that. All it sees is a page that has been modified by Linux and needs to bring it out to paging space. So over time you would have touched all slots in your VDISK and have CP back them with paging disk. If you have a stack of multiple disks (with different swap priority) you force Linux to re-use free slots before moving to the next VDISK. The total footprint of the VDISKs stays lower, and the locality of reference makes it more likely the swap slots remain in storage. Note that this is different from what people do with multiple swap partitions to achieve high bandwidth (since you have them with the same priority in that case).
The approach with 10 x 1 MB is a bit simplistic. I have much more complicated unproven suggestions ;-) What I have been recommending is to double the size of each next level of swap disk. The idea is that when you overflow one, you probably overflow the next one too. Your performance tool can show you where the contents of the vdisk are, and how much of the defined size is allocated. You could review those numbers over time and adjust the size of the swap disk layers to match the memory utilization levels.
Rob
PS I thought we had this in the Domino Redbook but I cannot find it right now.
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