Question: I wounder how important it is to have an UPS for a server. Everyone seems to use UPS for servers but not for client computers.

In example, if I use Windows Small Business Server on a small network for Active Directory and storing the users home directory for backup is it still useful with an UPS, why? If a power outage happens, shouldn't the filesystem handle that, so no files are corrupted? And in this case I think the problem is the same on the clients.

I do understand that it can be useful on servers when changing the server configuration, but that is done during a very limited time and it sounds like it's more worth to do good backups before the configuration.

And when thinking about database transactions, I think that the transaction logic in the database management system should handle it, that's what transactions are for.

And finally, if an UPS is used, doesn't the server applications need to have support for it? outherwise the power outage is only delayed? Or is it only the operating system that needs the UPS?

### Why is an UPS needed for a server but not for clients?

Answer: 
The brief interruption to someone's work is more likely going to cost less than buying and maintaining a desktop UPS. That's one of many reasons why many professional solutions are server based and centralized, as the service level can be focused there.

If there is a business justification with Return On Investment (ROI) in your environment, I would suggest a cost analysis between providing generated or battery backed power to the entire building versus buying individual UPSes for every workstation.

A server does not necessarily need to "support" a UPS, it's just a battery that's inline with the power source. Any "support" is going to be automatic shutdown or taking a preferred action in case of power loss.

The idea is to protect the equipment by conditioning the power as well as allowing the equipment to be properly shutdown in case of an extended outage. Depending on your budget, it is also often to maintain availability in case of power loss but with that service level you often have power generation in addition to a UPS.

A UPS is in addition to backups and has no direct correlation to backups beyond being a best practice within properly run IT infrastructure.

Without a UPS you could lose data, have hardware failure, and have system availability interrupted. It is often easier to find the ROI here than with workstations.
There are many, many reasons to use a UPS, but using your example of DB transactions, look at it this way. Even if a transaction is committed, you lose power, and your RAID write cache (you *are* using battery-backed write cache, correct?) batteries aren't functioning properly, you're out of luck.

Additionally, power outages are notoriously hard on equipment. Replacing a $500 workstation is one thing, but it's much more painful (both financially and otherwise) to replace a server that costs an order of magnitude more (data on that server notwithstanding). Most good UPSes also are line-interactive and will offer much better protection from power surges and brownouts than you'd get with a cheap power strip.

Overall, it's just a very well-accepted best practice.

> 
> I do understand that it can be useful
>  on servers when changing the server
>  configuration, but that is done during
>  a very limited time and it sounds like
>  it's more worth to do good backups
>  before the configuration.
> 
> 
> 

What does using a UPS have to do with changing a server's configuration?