Re: raid1 performance

[Justin Cormack <kernel@street-vision.com>]

> Lemme see if I am getting closer.  
> 
> When reading the disk there will be head seeks necessary.  When there
> are two disks, each with its own complete copy of all the data, there
> is no reason to keep the two disks' heads in the same place.  If their
> heads are in different places, a read can be issued to the disk whose
> heads are closer to the desired location.

yes. Look at raid1.c: the code is quite clear. Older versions didnt.

> This then brings up two more questions:
> 
>   1. Does the OS even know where the heads are in a modern IDE disk?

Not really. But there is probably a vague correspondence. Especially if
you havent remapped any bad sectors.

>   2. Is "closer" any more finely grained than a binary
>      positioned/not-positioned?

I think so. You can see different performance regions on disks (ie they
are faster on the outside for example). You could of course write a program
to test seek times from different areas and build up a real locality map.
It might not be worth it though.

> And I guess another question: How much does RAID 1 help and under what
> kinds of usage?

the latency is noticeably less in some cases, as the seeks should be smaller
on average. I have found this useful sometimes.

Justin