>> I think monitoring swap comes down to two things: strange memory use and >> whether there is a lot of stuff going in and out of swap and the amount >> of data. For I/O intensive servers, you don't want to see swap being >> used so monitoring swap usage can help to fine tune no. of processes or >> whatever to keep the box from using swap if it does use swap. >> > What your really saying, and this is a very fair answer, is to monitor > it on systems with different loads and learn what it should look like > when things are well, such that I can determine what is the norm, and > thus with that defination identify aberrations. No. You do not have to compare different systems. See below. > >> So I personally only care about swap if using it gets in the way >> (usually servers) and if my box does not have enough RAM for the >> applications I have to open where disk I/O is not critical (usually >> desktops) > Presently most of our servers have between 4 and 8 gigs of ram, and > our distro (a one off from CentOS) blankedly creates a 2 gig swap area > on top of lvm on top of a raid 1 md device. But with all the > applications we write not a one of them do we ever want to actually > use swap, we mainly have it there just in case. What we never did was > the research on our own to figure out what real threshholds exist such > that we could monitor and alarm when thresholds were crossed. On > Solaris we were given very specific things to look for by Sun, so we > were taken aback when RedHat (whom we were paying too) did not give > such specific instructions. It comes down to whether the use of swap has a significant effect on your operations. You can tell the kernel to not ever use swap unless absolutely necessary: swappiness = 0. Normally it is set at 60 (range is from 0 - 100). Let me explain how I would handle swap. Take a case of mail servers with hundreds of processes that only have 1GB of RAM and a pair of disks. I would use vmstat to check the 'si so' columns to see whether the box is swapping. Positive figures in both columns indicates use of swap and if I see constant reports of positive figures for these two columns, I start reducing the number of processes allowed to run to increase performance. This is how I identify active swap usage as opposed to swap in use. Swap in use is not the same as active swap usage. I could get maybe tens or even hundreds of megabytes being reported in swap but if vmstat 'si so' columns report zeros, I don't worry about its effect on disk i/o because there is none. However, if the amount of swap in use continues to grow over time, I start looking for memory leaks.