UNIX for Dummies Questions & Answers

hi guys

the monitoring team is using a tool for monitoring linux boxes and they set an alarm for swap memory to 10%(critical) I really has no idea when swap memory usage is high....

Can someone recommend me a threshold for this? when is warning or critical and this parameters can affect performance in my Suse Linux boxes?

for instance 1 box has been having "peaks" of 14% during last days....they don't know if the box has issues but they see the alarms and today another box had a peak of 64% and they are worried should they?

thanks a lot

Depends alot on the size of the swap and not so much % ratio... (what is 10% of 1 GB of disk to write nowerdays? 10% of 64 GB is not quite the same...)
The swap usage becomes a concern when performance is truely impacted ( really noticeable...). What is more important is the ratio of swap vs true RAM...
You can use vmstat to see what is going on...

thanks a lot, ok for example let's say 4GB physical RAM and 2 GB swap in your case what would be a warning and critical usage?

and second in vmstat what parameters should I be checking?

thanks a lot

I think the best way to answer your concern is:
Should a system swap? Only when under heavy usage?
- This should be episodic...If it more than weekly, then consider investing in more RAM.

Can the way swap is configured have an impact on performance when swapping?
- Yes, device swap is preferable, only use file swap when can not do anything else...
Then, if more than on disk, swap between all disks is far better than one big one on OS disk... e.g. Two 1 GB swaps on 2 disks is better than 2GB on only one disk
I had a system that always used swap (7 oracle instances running plus 1 sybase..) because it had only 2GB RAM since at the time it costed $$$$...
It had permanently 500MB of active swap with un-noticeable performance impact...
It had 4X1GB swap on 4 separate arrays on 2 separate contollers...

If you create 2 swaps to optimize swap then you must make them same size, in which case it will use both equally... otherwise the biggest is used first...