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Column
Meaning
total
The total amount of RAM installed on the server.
used
The memory that is used (from any source). This is calculated as follows: 
used
 = 
total
 - 
free
 - 
buffers
 - 
cache
.
free
The memory not being used by anything, the cache or otherwise.
shared
The memory used by 
tmpfs
 as well as other shared resources.
buff
/
cache
The amount of memory being used by the buffers and cache.
available
The memory that is free for applications to use.

---

Monitoring System Resources
[
 198 
]
You may have
 noticed in 
Figure 8.6
 that memory usage is also listed for a resource 
called 
Swap
. Let's take a look at that as well. We will dedicate the next section entirely 
to it so that we ensure we understand what it is, and what it does for us.
Managing swap
Swap
 is one of those things we never want to use, but always want to make sure 
is available. It's kind of like car insurance, no one is excited to buy it, but we do 
want to have it in case something bad happens. There's even some debate between 
administrators on whether or not swap is still relevant today. It's definitely relevant, 
regardless of what anyone says, as it's a safety net of sorts. (And disk space is 
cheaper nowadays, so dedicating some of our storage to this task isn't really a big 
deal, so we may as well).
So what is it? Swap is basically a partition or a file that acts as RAM in situations 
where your server's memory is saturated. If we manage a server properly, we hope 
to never need it, as swap is stored on your hard disk, which is orders of magnitude 
slower than RAM. But if something goes wrong on your server and your memory 
usage skyrockets, swap may save you from having your server go down. The 
Out 
of Memory (OOM) Killer 
may also activate itself when memory is full, to kill a 
misbehaving process that's using the majority of your memory, but as much as 
possible, we don't want to rely on that and instead ensure adequate swap in case 
memory is exhausted. 
The way swap is configured by default in Ubuntu has changed a bit since the time 
the first edition of this book was published. With Ubuntu 16.04 and earlier, a swap