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Turning Off the Computer

On Sat, 2008-10-18 at 21:13 -0400, Bruce Borland wrote:
> The other night I asked my son to turn off the computer.  He apparently 
> did not want to wait for the computer to shut down normally, so he 
> decided to switch off the power to the machine, stopping it immediately. 
>  I told him that was not good to do, but he asked me why.  I did not 
> have an answer for him.  He did this to our Windows machine, but I 
> understand that it is not good to turn off a Linux machine this way 
> either.  Can someone explain what problems are caused by such an 
> immediate shutdown?  I would like to know, and would like to tell my 
> son, too.  Thanks.

Hard disks are slow, so operating systems tend to cache data into
volatile system memory, before periodically flushing it out to disk --
only doing one larger batch write vs. lots of small writes all over the
place is a performance win. Pulling the machine in mid-flush means the
on-disk data winds up in an inconsistent state. In some cases, a hard
drive that looses power while in the middle of writing data can even
cause physical damage to the disk platters (rare anymore these days
though). Running services on the machine could be left in a funky state
too, if they were in the midst of some type of transaction (think
database apps here). Undoubtedly more examples out there.

Not that it shouldn't be possible to make shutdowns *much* faster for
desktop systems... I mean, if you've got a desktop not running any
services, and the user has closed all their docs, syncing in-memory
buffers out to disk and powering off is really pretty safe...


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