Subject: Disk Mirroring in Power5 Virtual I/O Server

Audience: All

Date: June 27, 2005

This tip discusses why you need a minimum of four physical disks to mirror virtual disks in a Power5.

Power5's virtualization allows you to "do more with less" by sharing disk and network hardware. It eliminates the need for each partition to have a dedicated boot disk/adapter, network card and associated I/O drawers and external switches. For example, my lab system has 1 shared boot disk and 1 network adapter and runs 6 Linux and AIX partitions concurrently.

The client's virtual disk physically resides in the VIO partition as a raw LV on a physical disk. Mirroring requires a minimum of 4 physical disks. This is because IBM recommends separating the VIO operating system and client LVs in different volume groups. Two disks will be needed to mirror the VIO operating system, and two disks for the client LV's. (page 8 in "Using the Virtual I/O Server" http://publib.boulder.ibm.com/infocenter/eserver/v1r2s/en_US/info/iphb2/iphb2.pdf ),

A final consideration is that the VIO server can not mirror client LV's. The mirroring must be done in the client operating system. So you would create two LV's on separate physical disks in the VIO server. On the client partition, you would use AIX's "mirrorvg" command to do the mirroring. (There is a "mirrorios" command to mirror the VIO operating system. However, is does not support mirroring the client LV's.)

For redundancy, I recommend two VIO servers. Each VIO server would have a minimum of 2 disks: one for the rootvg and the other for client LV's. In total, you still need a minimum of four disks. Mirroring within the VIO server is not required by virtue of having redundant VIO servers. Each client would have two disks: one in each VIO server. The client would be responsible for mirroring its disks.