A Look at AIX Mirror Pools

Edit: This is still an interesting method to consider.

Originally posted January 28, 2014 on AIXchange

A storage guy I know went to last fall’s IBM Technical University conference to learn more about IBM Power Systems and AIX, but he came away very excited about the AIX Logical Volume Manager (LVM). We may take it for granted, but for him this information about what we could do with our built-in volume manager was revolutionary.

In addition to the base LVM and its capability to easily mirror logical volumes (including mirroring physical disks to LUNs, as well as mirroring LUNs that might be coming from different physical storage arrays), there’s also the relatively new concept of AIX mirror pools.

IBMer Michael Perzl authored and recently updated a document that does a great job of explaining this. From the Introduction to AIX Mirror Pools” abstract:

            “This document tries to shed more light onto AIX mirror pools which were introduced with AIX V6.1 Technology Level 2. AIX mirror pools unfortunately seem not to be well known despite being a very powerful new AIX feature which simplifies the task of mirroring data significantly. One reason may be that for using AIX mirror pools no extra commands exist but the existing AIX LVM commands have been extended to incorporate the mirror pool functionality.

            “This document is not meant to be an all-encompassing guide to AIX mirror pools but give a first impression what tasks can be accomplished much easier than before. The intended audience for this document are AIX users and system administrators. A general knowledge and understanding of AIX LVM is required.

            “An example of how mirror pools can be beneficial is when used with remote disks. If a volume group is created with physical volumes that are located in two different locations, the disks in one location can be assigned to one mirror pool and the disks in the other location to a different mirror pool. When a logical volume is created in that volume group, each mirror copy of that logical volume can be assigned to a mirror pool. Thus, when partitions are allocated for that copy they will only come from disks that are in the assigned mirror pool.

            “Without mirror pools, the only way to restrict which physical volume is used for allocation when creating or extending a logical volume is to use a map file. This typically is a very tedious and error-prone process. Thus, the main advantage of mirror pools is that they simplify the task of mirroring data significantly compared to the steps that were required before. This is specially beneficial when used with remote disks. If a volume group is created with physical volumes that are located in two different locations, the disks in one location can be assigned to one mirror pool and the disks in the other location to a different mirror pool. When a logical volume is created in that volume group, each mirror copy of that logical volume can be assigned to a mirror pool. Thus, when partitions are allocated for that copy they will only come from disks that are in the assigned mirror pool.

            “The following system requirements must be fulfilled for mirror pools:

            • Mirror pools are only available in AIX V6.1 TL 2 and higher

            • Mirror pools are only available for SVG type (scalable) volume groups.

            • After assigning PVs (physical volumes) to a mirror pool, the volume group can no longer be imported to a previous version of AIX that does not support mirror pools.

            • While it is possible to assign multiple logical volume copies to a mirror pool, it is recommended that only one copy of a logical volume be assigned to a mirror pool.

            • Volume groups can enable strict mirror pools. If this is enabled all of the logical volumes in the volume group must use mirror pools.

            • Any changes to mirror pool characteristics will not affect partitions allocated before the changes were made. The reorgvg command should be used after mirror pool changes are made to move the allocated partitions to conform to the mirror pool restrictions.”

The entire document is well worth your time. Go through it and then and get onto a lab system so you can play around with mirror pools.