method allows schemes to reject the ctl request, pre-check
the parameters and/or modify/set parameters. There are 2
use cases that triggered the addition:
1. When implementing a R/O scheme, deletes will still
happen to the in-memory representation. The scheme is
not involved in that operation. The pre-check method
can be used to fail the delete up-front. Without this
the write to disk will typically fail, but at that
time the delete already happened.
2. The EBR scheme uses a linked list to record slices.
There's no index. The EBR scheme defines the index
as a function of the start LBA of the partition. The
add verb picks an index for the range and then invokes
the add method of the scheme to fill in the blanks. It
is too late for the add method to change the index.
The pre-check is used to set the index up-front. This
also (silently) overrides/nullifies any (pointless)
user-specified index value.
partitioning class that supports multiple schemes. Current
schemes supported are APM (Apple Partition Map) and GPT.
Change all GEOM_APPLE anf GEOM_GPT options into GEOM_PART_APM
and GEOM_PART_GPT (resp).
The ctlreq interface supports verbs to create and destroy
partitioning schemes on a disk; to add, delete and modify
partitions; and to commit or undo changes made.
