Re: hard drive impact on overall system performance

Previously steve.anon@xxxxxxxxx wrote:
> Hi guys, I consider myself a decent computer tweaker, but recently
> something I had learned to live with has bugged me - and I'd like to
> fix it. Maybe you can help.

> Let me explain: my current box is a p4 3Ghz, 1Gig of ddr2 ram, win xp.
> The hard drive is 180gig SATA. It plays the latest video games, etc, no
> problem. But, say I unrar a 600 meg file, that's when I can hear the
> drive go in overdrive. Fair enough as it's probalby reading and writing
> at the same time, but that doesn't explain why the machine slows down
> to crawl when that happens.

> What I don't understand is why it slows down at all. After all I'm
> using only 200meg of ram, so there's no reason for swapping. Unraring a
> file itself takes very little ram if any. No, it seems to be just
> 'using' a drive that somehow overload the bus and slows down the rest.

> Now, the issue is common to all computer I ever owned or worked on
> (including my good ol' 386). Basically, I build them with the fastest
> components of the time, yet every single box had the hard drive as a
> bottle neck to performance.

> Is there a way to somehow 'fix' this issue? Or is this just the way
> things work?

It cannot really be fixed. The problem is that modern OSes di write
buffering. At some time the buffers age enough that a foced flush is
needed. This means the writing gets priotity over every other action,
hence the slowdown. One thing you can do on a decent OS is mount
the disk as "suncronous", causing the OS to not do write buffering.
This degrades performance but increases responsiveness for other tasks.

The problem is not so bad with better I/O sheduling. Also if you read
from one disk and write to anothyer you get significantly less head
movements in the disks, and these are still slow.

Arno
.