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: 
Accidents do still happen. RAID cache batteries not working properly is a concrete example, but journaling filesystems and transactional integrity simply aren't guaranteed when the power is pulled.

Plus, any UPS worth a damn also does line conditioning to protect the hardware from current and voltage variations, extending the lifetime of your hardware.

Why servers and not desktops? Some places do, but it's expensive. How much is the data on any client worth? How much is the data on the server worth?
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?