EBS 12.2 -- Solaris Apps Tier, "setting the Swap Space" , "/tmp on swap"

According to some administrators, swap should not be used in a production system.. They may say "if we are on swap(using swap at any time), we ' re screwed..

With this in mind, they may not configure a swap at all or they may give you a very small swap area for the use with your applications.

In case of EBS 12.2, this is especially true for the apps tier node. In apps tier, the swap area is generally kept in the minimum size.

On the other hand, I don't find it true.

Let's take Solaris for example..

In EBS 12.2 Solaris installation document, there is no info about swap space.

That is, in EBS 12.2 Solaris installation document, there is only one swap space related requirement and it is lead to the database tier..

Interestingly, in EBS 12.2 Linux installation document, we have a line saying "it is recommended that the swap space on the system be 16 GB or more

Well.. for Solaris actually, this info is more crucial..

In Solaris 10 and 11, there is a important enhancement which is made for the temporary file performance. That is, in Solaris 10 and 11, /tmp is mounted on swap.

Here is an "mount |grep tmp" output, which was get from a Solaris 11.3;

/tmp on swap read/write/setuid/devices/rstchown/xattr/dev=90c0002 on Wed Oct 11 13:42:55 2017

You see /tmp is on swap!

So, when you use /tmp for storing some files, your swap is used in the background.

This means, if you configure a small swap area, you may end up with application errors.

Programs like FNDWRR.exe will definetly crash randomly and even your OS commands like top, like ls will hang during these peaks.

When FNDWRR.exe crashes, you won't be able to see any concurrent output.

So, what I recommend is, to set the swap size to a value which is equal or greater then your /tmp directory size. 

Ofcourse, if you have unnecessarily big /tmp directory and if your EBS application code uses a very little portion of it, then you can arrange your swap size accordingly.

Bytheway, you may change this behaviour on OS level.. On the other hand; as this /tmp on swap thing comes by default, I don't want to make recommendation on Solaris OS layer..

Anyways.. In conclusion, we use swap and we use it very often in Solaris..

Lastly, the important point is ; using swap is not a bad thing for all the cases..  It doesn't always mean that we are screwed:)

Here we see an enhancement to store the temporary files on swap, rather than on disk and this increases the speed of accessing to those files.