Promise RAID-1 vs software RAID-1

Christopher Chan cchan at
Wed Jun 16 23:12:21 UTC 2004

Jack Howarth wrote:
>     I have a couple questions about the finer details of
> the Promise RAID-1 vs the use of software RAID-1 with
> the promise-sata driver. We have two machines here, one
> of which has RedHat 9 and the Promise RAID driver installed
> for a RAID-1 mirror. The second machine has Fedora Core 2
> with the promise-sata driver used for a software RAID-1
> mirror. The question has to do with any speed improvements
> that the Promise RAID-1 might have on reads. One of our
> users has been told that the Promise RAID-1 actually reads
> off of both drives at the same time to achieve a speed
> increase on reads (by splitting the reads). Is this true
> and if so does the software RAID-1 attempt something like
> this? I suspect this isn't the case and they are confusing
> RAID-1 with RAID 0+1. My understanding was that a RAID 0+1
> required 4 drives and wasn't possible with just two drives.
>     If it is true that the Promise RAID 1 does in fact
> read partially from each drive to increase the throughput,
> does software RAID-1 do the same?

Don't know about the Promise software raid driver but the Linux kernel 
does give you the ability to do simultaneous reads from both drives in a 
  Linux RAID 1 mirror pair.

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