On Thu, Sep 05, 2002 at 02:18:12PM +0200, Ismael Touama wrote:
> > -----Message d'origine-----
> > De : [EMAIL PROTECTED]
> > [mailto:[EMAIL PROTECTED]]De la part de Kent Borg
> > I have a motherboard with two regular IDE controllers.  I put a hard
> > disk on each, and set up a software raid that will boot from these two
> > disks and run as raid 1.
> >
> 
> As far as I know one controller is made for one *or two* disk.
> The second is for CD-ROM or devices like that (not more that two too).

Yes, but I also said that raid 1 offers a speed advantage, and for
this you want only one disk per controller.  It works this way: On
writes each disk is writing the same information (so you have
redundant copies) and if they are on different controllers, they can
both be writing at the same time, so writes are essentially the same
speed as non-raid.  On reads, each disk can be busy reading stuff for
different processes, both at the same time, making reads faster.
(Even for a single process, the system can choose the disk whose head
is likely closer to the correct location for faster seek times.  Kinda
like a two-deep cache of head positions.)

If you put both raid 1 disks on the same controller you lose the
advantage of keeping both disks active at the same time.  (Also, it is
possible that there are some disk failures that would crash the
machine with two disks per controller that would not crash a machine
with one disk per controller.)

All that said, I also have a CD-ROM in the system.  I put it on a
controller with one of the hard disks, which means that when the CD is
in use for data I will get a significant performance degradation with
my raid 1.  But mostly the CD just sits there doing absolutely
nothing, and causing no delays.


-kb



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