Why does rootvol have a SD labeled "rootdisk-B0"?

东方蜘蛛

A quick overview:
The B0 subdisk is there as a protective mechanism when an encapsulated file system begins at cylinder 0 of the disk.  Since VERITAS Volume Manager (VxVM) does not allow access to the bootblock (first sector) of a disk, it must relocate that information somewhere else; thus it creates the B0 subdisk as a replacement for the area of the volume that would normally be on the bootblock sector.  
DO NOT REMOVE THIS SUBDISK!  It is vital for the integrity of the rootvol and can be difficult to remake if it is deleted.
A more detailed overview:
File systems are built to not interfere with the bootblock.  They generally do not touch any data until 8k into the device, where they store their superblock.  Applications that use raw partitions, however, do not make the same concessions.  They tend to want to use all of the space.
Since the space allocated by VxVM can be dynamically re-adjusted and it is never possible to rely on a file system being on that particular area of the disk, VxVM must protect that region.  It does this by shifting the start of the Public Region by one sector, preventing VxVM from ever using the first sector of the disk.  
The file system that started on cylinder 0, however, still needs to be able to reference that sector so that all of its offsets will be in line. To facilitate this the B0 subdisk is created and placed onto the beginning of the volume as a replacement for the bootblock that can no longer be accessed.
When looking at the VTOC of an encapsulated disk, it can be seen that the Public Region (tag 14) is defined the same as the BACKUP slice, or the entire disk (slice 2). This gives the impression that the Public Region is the same size as the disk.

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