Birmingham mumbai

[
 197 
]
At first glance, it may appear as though this server has only 
179
 MB free. You'll see 
this in the first row and third column under 
free
. In actuality, the number you'll 
really want to pay attention to is the number under 
available
, which is 
1613
 MB in 
this case. That's now much memory is actually free. Since this server has 
1987
 MB 
of total RAM available (you'll see this on the first row, under 
total
), this means that 
most of the RAM is free, and this server is not really working that hard at all.
Some additional explanation is necessary to truly understand these numbers. 
You could very well stop reading this section right now as long as you take away 
from it that the 
available
 column represents how much memory is free for your 
applications to use. However, it's not quite that simple. Technically, when you look 
at the output, the server really does have 
179
 MB free. The amount of memory listed 
under 
available
 is legitimately being used by the system in the form of a cache but 
would be freed up in the event that any application needed to use it. If an application 
starts and needs a decent chunk of memory in order to run, the kernel will provide it 
with some memory from this cache.
Linux, like most modern systems, subscribes to the belief that "unused RAM is 
wasted RAM." RAM that isn't being used by any process is given to what is known 
as a 
disk cache
, which is
 utilized to make your server run more efficiently. When 
data needs to be written to a storage device, it's not directly written right away. 
Instead, this data is written to the disk cache (a portion of RAM that's set aside) and 
then synchronized to the storage device later in the background. The reason this 
makes your server more efficient is that this data being stored in RAM would be 
written to and retrieved faster than it would be from disk. Applications and services 
can synchronize data to the disk in the background without forcing you to wait for it. 
This cache also works for reading data, as when you first open a file, its contents are 
cached. The system will then retrieve it from RAM if you read the same file again, 
which is more efficient than loading it from the storage volume each time. If you just 
recently saved a new file and retrieve it right away, it's likely still in the cache and 
then retrieved from there, rather than from the disk directly.
To understand all of the columns shown in 
Figure 8.6
, I'll outline the meaning of each 
in the following table: