Received a question about whether you can use software RAID5 across multiple hardware RAID5 or RAID0 storage systems using iscsi.

In the UNIX world, your scenario is fairly routine, and often referred to as “plaiding” (stripes perpendicular to stripes.) It is typically done to increase aggregate bandwidth to the volume as well as to create larger volumes than optimal (or even possible) on the hardware RAID device. This is done on block devices with RAID subsystems, and can be accomplish much in the same way with iSCSI, but there are a few caveats.

In all cases, the storage subsystems must have their own RAID capability.

In the case of FC, each Storage subsystem, involved in the plaiding will have IO controllers with RAID capability; can’t use a JBOD here.

In the case of iSCSI – such as a NAS Server Appliance or an iSCSI array– will have its own RAID capability as well. These are usually configured by default, as RAID 5 or even RAID 6 or RAID 10. The appliance can be reconfigured to RAID 1 or RAID 0 if you choose. Just keep in mind, if you choose RAID 0, a single disk failure on a unit will take the entire server for that data segment out. Also, on appliances with real deep storage capacity i.e.: 1TB +, I can’t imagine how long it would take to resync it. I’d leave it RAID 5 for a two-tiered redundancy scheme.

Also depending on what host you use, RAID 5 on the host would not be your performance choice. Personally I’d choose RAID 1, as it is very low overhead on the host–side.

Since the question didn’t mention what host operating system they wanted to use in this layered RAID scenario, I will cover both UNIX and Microsoft.
In UNIX, the host would need a volume manager capable of doing software RAID, something like: Veritas Volume Manager (or should I say Symantec Storage Foundation, now) Veritas Volume Manager is a good reference and supports plaiding across multiple LUNS. Here is how it is set up: An SCSI LUN would be created on each NAS Server or iSCSI target. The Sun iSCSI initiator or the UNH initiator would work for attaching the iSCSI LUNS to the host. Once they can be seen by the host’s disk layer, the volume manger would allow you to RAID across these LUNs.

In Microsoft land, the process is similar, however in order for you to use Microsoft’s SW RAID capability, built into Windows Server 2003 for instance, iSCSI LUNs have to be configured as dynamic disks, which are currently not supported. Bummer…Microsoft has a plethora of FAQ’s and information on their web site warning people to not use dynamic disks on iSCSI LUNs with the standard Microsoft Volume Manager. I have personally tested with it setting up a RAID 1 (mirror) on the host and it works- fails over / fails back just fine. I have never had any issues with the LUNs not attaching at start up as Microsoft claims is the liability; but then again, I am not using this in a production environment so I my testing is not absolute.
Doing some research on the web, VERITAS Storage Foundation™ 4.3 for Windows does support Dynamic disks. The standard Volume manager in Windows does not.

This entry was posted on Monday, December 18th, 2006 and is filed under Storage Applications, Advisor - Steve Rogers. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.