Performance impacts of mirroring rootvg

In mirroring, when a write occurs, it must occur to all logical volume copies. Typically, this write takes longer than the logical volume that is not mirrored.

Although mirroring is common for customer data, particularly in database environments, it is used less frequently for system volumes.

Mirroring can also cause more processor overhead, because two disk I/Os take more instructions to complete than one. It is important to understand the layout of the rootvg logical volumes so one can guess where problems might exist when mirroring the rootvg logical volumes.

The logical volumes that are found in rootvg that includes the files in /, and the heavily used /usr/bin file where many executable programs exist, must be read-mostly data. The paging space must have writes only if the amount of physical memory in the system is insufficient to hold the current level of activity. It is common for systems to page from time to time, but sustained heavy paging usually leads to poor response time. The addition of physical memory generally resolves this issue.

The /tmp and /var file systems do see file-write activity for many types of applications. Applications, such as the compiler, often create and write temporary files in the /tmp directory. The /var directory receives files that are assigned for mail and printer queues. The jfslog is write only during normal operation. In the remaining file systems, only the /home directory is active during normal operation. The user home directories are frequently placed in other file systems, which simplifies rootvg management.

The rootvg can be mirrored by mirroring each logical volume in rootvg with the mklvcopy command or by mirroring the entire volume group by using the mirrorvg command.

By default, the mirrorvg command uses the parallel scheduling policy and leaves write-verify off for all logical volumes. It does not enable mirror-write consistency for page space. It does enable mirror-write consistency for all other logical volumes. Place the logical volumes that are frequently written close to the outer edge of the disk to minimize the seek distance between the logical volume and the mirror-write consistency cache.

The mirroring rootvg does not significantly affect the performance that is if paging space is mirrored, the slowdown is directly related to the paging rate. So systems that are configured to support high paging rates, with paging spaces on rootvg, might not want to implement rootvg mirroring.

In summary, mirrored rootvg might be worth considering when your workload does not have high sustained paging rates.