14.2 Why Do I Need LSM?

Since LSM is a software RAID implementation, it is an inexpensive alternative to hardware-based RAID. Although LSM has some limitations that prevent you from implementing a complete no-single-point-of-failure (NSPOF) solution, as we will discuss in the next section, it can still get you the redundancy you need in the most important places: your data and the cluster-common file systems. Furthermore, if you are migrating from a previous TruCluster configuration (V1.5 or V1.6) and it is a NSPOF cluster, then you are using LSM (it was the only way to achieve a NSPOF solution because multiple paths to storage devices were not supported until Tru64 UNIX version 5.0).

The fact that LSM continues to be supported gives you another option for implementing redundancy. Coupled with a LAN-based cluster interconnect, you can put together an inexpensive, redundant, and highly functional cluster. Starting in TruCluster Server version 5.1A, you can use LSM to mirror the cluster_root, cluster_usr, cluster_var, swap partitions, and, of course, your data. What this means is you now have a low cost cluster solution. Prior to V5.1A, cluster_root and swap could not be brought into LSM.

As much as we love multiple bus, multiple switch, Fibre Channel RAID controller solutions, not everyone can afford a storage solution in that price range. So this is a welcome alternative to getting redundancy via software-based RAID. While it is not a completely redundant solution, it is close. The only pieces missing from the fully redundant pie are the quorum disk and two of the member's boot disk partitions (the boot_partition and cnx partition). Of course, one could argue that the purpose of the quorum disk is for additional redundancy, and it, in and of itself, is not a single-point-of-failure (SPOF). And the loss of one member's boot disk would not cause a cluster-wide failure, so it too is not a SPOF. Furthermore, replacing a quorum disk or member's boot disk is not a very difficult or time-consuming procedure (a process we will cover in Chapter 22).

If you already use LSM on your current pre-V5.0A cluster, then migrating any data you have on LSM volumes can be accomplished with a minimal downtime.

Note

In order to enable many of the advanced LSM features (mirroring, striping, etc.), you will need to install the LSM-OA license product authorization key (PAK). This is an additional cost license, so unless you have it, you will have to contact Compaq or a reseller to get it.