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RAID 1 with three drives

Posted on 2007-08-05
Medium Priority
Last Modified: 2013-11-14
I am looking for a SATA PCI Express RAID controller that will do a RAID 1 configuration with three hard drives.  We don't want to do RAID 5, 6, or 10 because we do not want any striping or parity drives.  We went a 1x write and a 3x read system.  With a normal RAID 1, the controller mirrors the data on two drives, but we want the data mirrored onto three drives.  We also want the ability to, if the RAID is degraded, to take out a single drive and plug it into another computer to recover data.  We are currently looking at the PROMISE SuperTrak EX8350 PCI-Express x 4 SATA II Controller Card (NewEgg Item # N82E16816102071 if anyone is interested).  If someone could please provide me with insight that would be much appreciated.
Question by:s2000lover
LVL 34

Expert Comment

ID: 19635524
Unfortunately there is no way to mirror 3 drives with a hardware raid solution you would need to use a software raid solution. Here's a link to a question similar to this one.

Accepted Solution

eagle0468 earned 2000 total points
ID: 19635643
You won't be able to configure RAID 1 with three drives.  What you can do, however, is configure two drives with RAID 1 and have the third drive set as a hot spare (normally termed a "global hotspare" on most RAID controllers).  This will give your array the redundancy you are seeking in addition to adding data integrity with your hotspare, which will automatically begin to rebuild should one of the drives in the mirror fail.  RAID 1 will give you a high level of read/write performance because both drives are read to and written to simultaneously.  RAID 10 is the highest performance RAID solution in relation to cost.  If you have the budget and the desire for performance you might wish to add two more drives and stripe two mirrors into a RAID 10.  With the additional drive as a hotspare you will have optimum performance and data integrity.
LVL 39

Expert Comment

ID: 19636089
You might misunderstand the raid 1 seriously. There is no third drive for this raid level.

RAID 0+1: Striped Set + Mirrored Set (4 disk minimum; Even number of disks) provides fault tolerance and improved performance but increases complexity. The key difference from RAID 1+0 is that RAID 0+1 creates a second striped set to mirror a primary striped set. The array continues to operate with one or more drives failed in the same mirror set, but if two or more drives fail on different sides of the mirroring, the data on the RAID system is lost.
RAID 1+0: Mirrored Set + Striped Set (4 disk minimum; Even number of disks) provides fault tolerance and improved performance but increases complexity. The key difference from RAID 0+1 is that RAID 1+0 creates a striped set from a series of mirrored drives. The array can sustain multiple drive losses as long as no two drives lost comprise a single pair of one mirror.
LVL 14

Expert Comment

ID: 19636125
I've not seen a pc based hardware solution for your problem.

The mirroring software that comes with Sun Solaris would allow you to do exactly what you want.

I actually use a script to mirror a system snapshot to  a third drive.

>>We also want the ability to, if the RAID is degraded, to take out a single drive and plug it into another computer to recover data.

Disk Raid groups that provide protection, do not provide data recovery, that is what backups are for.  

A mirror is exactly that, full copies of the data, no matter how many drives you have.  So, if the data is corrupt, its corrupt on all copies.

If this is production data and  very important, it may be time to move beyond pc solutions and start looking at SAN solutions.

Good Luck!
LVL 56

Expert Comment

ID: 19637662
You can have three or more disks in a RAID 1 array but the only controller I know of that ever implemented it is the DEC HSG80, that went up to 6 copies of the data and was intended for very fast read.

but I wouldn't bother to read it since you can hardly put one in a PC.

HP-UX also allows 3 mirrors like Solaris that arthurjb mentioned. There's also some Windows software that maintains multiple mirrors although I think it only reads from the second and third disks if the first one fails.

The other option is to create two RAID 1 arrays and them mirror them (RAID 11?)

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