Archive for the ‘Storage’ Category

Dell PowerVault MD3000i balancing virtual disks between RAID controllers

Friday, September 17th, 2010

The Dell PowerVault MD3000i iSCSI SAN has a pair of redundant RAID controller modules designed to maximise availability of the mission critical data held in your Storage Area Network. These RAID controller modules can work in a pseudo active/active mode through a bit of a work around, however there seems to be some confusion on the Dell mailing lists and forums as to exactly what is required in order to balance the load across the two RAID controller modules.

There are references to creating two sets of both the physical disks via the disk groups (the actual RAID sets) and the virtual disks (the part of each RAID set exported as a LUNs) in order to split the load between the two RAID controller modules, as each can only be active on a single RAID controller module at once.

It turns out that it isn’t necessary to create separate disk groups in order to split the physical disks across the two RAID controller modules, as each RAID controller module has access to all of the disks in the system and both of the RAID controller modules can talk to the same disks at once.

Instead, what you should do (if possible) is two create a pair of virtual disks in your disk group (or multiple disks groups if you want to utilise different RAID levels for different virtual disks). When more than one virtual disk exists, the Dell PowerVault MD300i will automatically balance the virtual disks across the RAID controller modules in a round robin fashion in order to try and spread the read/write load between the two RAID controller modules.

Although each RAID controller module is master for it’s own virtual disk, it can still pick up the virtual disk(s) off the other RAID controller module should one fail and so still provides a redundant, highly available storage system.

You can view the details of how the virtual disks are currently distributed across the RAID controller modules by going to “Support” and then “View Storage Array Profile” in the Dell Modular Disk Storage Manager software. The “Virtual Disks” tab lists the “Preferred owner” as well as the “Current owner” for each virtual disk.