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SubjectRe: raid1 performance
On Wed, May 01, 2002 at 11:23:23PM +0200, Bernd Eckenfels wrote:
> In article <20020501130127.A10936@borg.org> you wrote:
> > 1. Does the OS even know where the heads are in a modern IDE disk?
>
> > 2. Is "closer" any more finely grained than a binary
> > positioned/not-positioned?
>
> > And I guess another question: How much does RAID 1 help and under what
> > kinds of usage?
>
> No, you just distribute the ready round robin, this means each disk has only
> half the seeks it had before.

No, this is the way it was done a long time ago.

It turns out to be an incredibly bad idea. In fact, it is the most CPU-efficient
way of guaranteeing the largest average seek times on your disks ;)

The RAID-1 code now looks at which disk worked closest to the wanted position
last, and picks that disk for the seek.

> As long as you do not spread continous blocks
> (readahead) stats are good you actually reduce overall seeks. This helps
> actually even if no seek is involved because of the fact that you need to
> wait for the begin of a track to read it.

The "new" code (which is not that new anymore) will allow one disk to keep
on a single sequential read for a long time (eventually it will kick in the
idle disk(s) though).

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