Capacity ?… I Think I’ve Heard Of That… part 1

Capacity, what is capacity ?

Well, it seems to be quite elusive; it’s normally spoken of as something we don’t have enough of, and yet when we try to measure it we seem to have too much ? How can this be ?

 

Perhaps a couple of definitions of capacity are pertinent[1]:

  • the maximum amount or number that can be received or contained.
  • reaching maximum capacity.

 

The key here is that capacity is finite, it doesn’t change.  If something can hold a maximum of 4 widgets today, it will  hold a maximum of 4 widgets tomorrow.  The only way to change the capacity is to physically change it’s size.  From our previous example, if I have 8 widgets then I need twice the capacity.  How, quickly I can use up the capacity, though, can be variable; quick or slow, steady or sporadic.  This is where performance comes in.

 

A definition of performance is:

  •  manner or quality of functioning[1]

 

That is to say performance is variable.  Think of athletes, they have good days and break World records, and bad days and come last.

To illustrated this further:

The capacity is the bucket, the performance is the tap.  If I need to fill the bucket within a certain time then it is the throughput of the tap that is key.  The bigger the tap, the quicker it fills the bucket.  The amount of water the bucket can hold, however, doesn’t change.

The same is true in IT; here the bucket is the server, the disk, the network.  How many widgets can we process, how many files can we store and how much data we can transport are capacity.  Why suddenly can we only process half the number of widgets, not write our files quickly enough or the internet is slow are more to do with performance.

 

 

[1] http://www.dictionary.com

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