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Boot-time volume restrictions

Volumes on the root disk differ from other volumes in that they have very specific restrictions on their configuration:

• The root volume (rootvol) must exist in the default disk group, bootdg. Although other volumes named rootvol can be created in disk groups other than bootdg, only the volume rootvol in bootdg can be used to boot the system.
• The rootvol and swapvol volumes always have minor device numbers 0 and 1 respectively. Other volumes on the root disk do not have specific minor device numbers.
• Restricted mirrors of volumes on the root disk device have overlay partitions created for them. An overlay partition is one that exactly includes the disk space occupied by the restricted mirror. During boot, before the rootvol, varvol, usrvol and swapvol volumes are fully configured, the default volume configuration uses the overlay partition to access the data on the disk.
• Although it is possible to add a striped mirror to a rootvol device for performance reasons, you cannot stripe the primary plex or any mirrors of rootvol that may be needed for system recovery or booting purposes if the primary plex fails.
• rootvol and swapvol cannot be spanned or contain a primary plex with multiple noncontiguous subdisks. You cannot grow or shrink any volume associated with an encapsulated boot disk (rootvol, usrvol, varvol, optvol, swapvol, and so on) because these map to a physical underlying partition on the disk and must be contiguous. A workaround is to unencapsulate the boot disk, repartition the boot disk as desired (growing or shrinking partitions as needed), and then re-encapsulating.
• When mirroring parts of the boot disk, the disk being mirrored to must be large enough to hold the data on the original plex, or mirroring may not work.
• The volumes on the root disk cannot use dirty region logging (DRL).

In addition to these requirements, it is a good idea to have at least one contiguous, (cylinder-aligned if appropriate) mirror for each of the volumes for root, usr, var, opt and swap. This makes it easier to convert these from volumes back to regular disk partitions (during an operating system upgrade, for example).