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.
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