Overselling and overloading are two terms often heard when discussing web hosting, especially shared and reseller hosting services. In this post, I’ll explain the meaning of these terms – nothing more, nothing less.
This term is easily explained using a practical example. I’ll use disk (storage) space to explain it, but the same principle goes for all the other hosting server resources.
I have a reseller account that allows for 15 GB of total storage space and 10 user sub-accounts (cPanel-s).
- Currently there are 5 websites hosted there.
- All the websites combined take 3 GB of storage space.
Now, each of the 5 websites could get 3 GB of storage, which would fill up my available 15 GB. However:
- websites can temporarily use more space – when creating backups, importing/exporting databases etc.
- Not all the websites are of the same size: some use a few hundreds of MB, while others use over 1 GB.
With the current situation, could I allow 5 GB of storage to each of the 5 websites (total of 25 GB), even though I have only 15 GB in total available? Yes. This is called overselling.
Why would I do something like that? Because it would allow all the websites to function properly. As long as they don’t take over 15 GB in total at the same time, it’s all good. Overselling is based on statistics. 100 websites take a certain amount of storage space on average. One website can be much larger, other much smaller, but on average 100 websites will (almost) always take a given average amount of storage space.
What if the calculation is wrong? It is smart to always calculate with a certain safety reserve – just in case.
What if, in spite of that, more resources are taken than there are available? In that case, that is called “overloading” and is explained in the next chapter:
“Moria. You fear to go into those mines. The Dwarves dug too greedily and too deep. You know what they awoke in the darkness of Khazad-dûm… shadow and flame.”
— Saruman, The Fellowship of the Ring
If you overdo with overselling, you get overloading of the available server resources. If RAM, CPU, storage, or other server resources are not sufficient for the requirements of the hosted websites, then websites will not work properly, not load fast enough, sometimes even not work at all (i.e. giving “Error 500” when trying to open a page).
For example I’ll take 15 GB of available server storage, with average usage of 1 GB per website. What happens if we place 15 accounts on such server, allowing 5 GB per account (cPanel)?
- Total used disk space would be the entire 15 GB (some websites using 0.5 GB, other 1.5 GB and so on, but an average of 1 GB per website).
- Every user would see they have 5 GB of available space. A user who’s website takes 2 GB of space, would see (and think) they have 3 more GB of storage available.
- Since all the available 15 GBs are used, no user could upload/create a website backup on the server, seeing an error when trying to do so – because the disk is practically full, even though the account allotted limit is not exceeded.
It can be said that overloading is in fact incompetent (irresponsible, greedy) overselling. While overselling in and of itself is a normal, rational, optimal way of using the available server resources, overloading is very bad.
All the hosting providers use overselling (except with dedicated servers), but the responsible ones that provide good quality service, don’t overload their servers. Cheap hosting providers often use “very liberal overselling” to say the least.