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SubjectRe: Disk Mirroring
On 12 Sep 1996, Mike Bristow wrote:
> >Can anyone tell me if there is a way of mirroring disks in Linux? We use
> >this approach at work to give our system online redundance so if a disk
> >starts to go bad, we break the mirror and take the offending disk/partitions
> >off line until the repair/replacement is done. Since all data is
> >duplicated on the mirror device, the user has no idea that a disk failure
> >has occured.
> >
> You're after RAID support. Doesn't exist (yet). Very Clever People
> are working on it...

Not quite true, hardware controller based RAID has been in use under Linux
for over 3 years now. Since then, the driver for the DPTs (EATA-DMA) has
been available.

> For the cuurent state of play, look into md.
>
> As I understand it, this is never as good as a special RAID controller.
> <simplifcation>
> This is a very clever box, which pretends to be a SCSI (or whatever) HDD
> while actually containing 9 (or whatever) real SCSI HDD's. As far as Linux
> is concerned, it's just got a SCSI HDD, and doesn't care any more.
> </simplifcation>

Yes, it is a little too much simplified (you're mixing external
SCSI--SCSI RAID subsystems with internal SCSI RAID controllers here) but
otherwise the idea is correct.

> These are normally better than a software solution, because you can do
> things like take out a broken HDD, and replace it, while the system
> is online.

True.

> They are, howvere, expensive....

But those who really need the IO performance and/or security are
willing to pay for it, since they know it is worth it.

Mike

Michael Neuffer i-Connect.Net, a Division of iConnect Corp.
mike@i-Connect.Net 14355 SW Allen Blvd., Suite 140
503.677.2900 Beaverton, OR 97008
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