Mirroring on a RAID 5 array

[Mark Rhodes]

I currently have a FC30 Nike array on a K series machine. All the LUNs on the array are set up at RAID 5. I need to mirror, using Mirror Disk UX, some of the logical volumes to logical volumes on another RAID 5 LUN. When I tried to set this up I was unable to merge and resync the mirrors after they were split. Does anyone know if this can be done on my current hardware configuration. If not is there another array subsystem that would support this functionality.

[Petr Cerny]

Hi Mark,
this is definitely supported and working. Check please patches related to mirrordisk software and lvm patches + dependencies.
The only correct way for installing mirrordisk on 10.20 is as follows: 1)install HP-UX itself 2) install mirrordisk sw as wery first product 3) install rest of applications 4) install all patches. If you change this order, mirrordisk sw could have "strange" behaviour.
What is (exactly) the error you are getting after merge?

regards,
Petr

[Mark van Silfhout]

Indeed after installing mirror-ux, you have to re-install the lvm patches (including dependencies).
Why would you create a (software) mirror of the RAID 5 volumes? (they are protected against hardware failures). If this is for backup reasons, you might look at a XP 48/512, which supports Bussiness Copies. (http://www.storage.hp.com/disk_arrays/index.html)

[Dave Wherry]

Like Mark I'm also curious why you want to mirror Raid-5? Raid-5 is slow on writes and you would be doubling the number of writes by writing to the primary and then the mirror.
It might be possible to improve read performance by mirroring. The read can be serviced from either of the disks which can reduce queuing the reads. Of course Raid-5 reads are pretty fast anyway.

[Mark Rhodes]

I know that mirroring a RAID 5 array to another RAID 5 array is a strange configuration. Unfortunately the database I run has to be backed up with no activity, so I need to have a mirrored copy that is split to avoid stopping my processing during backups. I need the hardware redundancy afforded by RAID 5 due to my location (Alaska) and the fact that I may not be able to get imediate repair service from HP. I appreciate the comments I have received so far. I will look to the install process of HPUX and Mirror Disk UX as well as look into the XP48. Again, thanks for the advice.

[Josee Bourget-Thuma]

Mark,

I can see why you would want mirroring if you are running an RDBMS. If you have enough space, I suggest you reconfigure the RAID 5 to RAID 1. Oracle recommends that you place your database on RAID 1 for example. Just remember you lose 1/2 the space. While you're at it, ensure you DO NOT enable write cache. This would cause data loss in the event of failure. Also, keep your mirrors on a seperate buses. If the array has more than one controller, you can configure your LUN's such that you can minimize I/O contention as well. You just need to bind each LUNs to the various controller individually. I'm currently doing this exercise for our databases and it's been worth the extra disk space. I use NIKE20 arrays.

Regards,
Josee