The weakness of the previous disk layout is that if either Disk 1 or Disk 2 fails, the whole database server goes down until you restore the dbspaces on these disks from backups. In other words, the disk layout is poor with respect to availability.
An alternative disk layout that optimizes for availability and involves mirroring is shown in Figure 52. This layout mirrors all the critical data spaces (the system catalog tables, the physical log, and the logical log) to a separate disk. Ideally you could separate the logical log and physical log (as in the previous layout) and mirror each disk to its own mirror disk. However, in this scenario, the required number of disks does not exist; therefore, the logical log and the physical log both reside in the root dbspace.