Writes in sequential write required zones must happen at the write
pointer. Even if the submitter of the write commands (e.g. a filesystem)
submits writes for sequential write required zones in order, the block
layer or the storage controller may reorder these write commands.
The zone locking mechanism in the mq-deadline I/O scheduler serializes
write commands for sequential zones. Some but not all storage controllers
require this serialization. Introduce a new flag such that block drivers
can request pipelining of writes for sequential write required zones.
An example of a storage controller standard that requires write
serialization is AHCI (Advanced Host Controller Interface). Submitting
commands to AHCI controllers happens by writing a bit pattern into a
register. Each set bit corresponds to an active command. This mechanism
does not preserve command ordering information.
Bug: 264714656
Change-Id: I1f64eb61571a66b99a0d83bef4438e2187c16f8e
Signed-off-by: Bart Van Assche <bvanassche@google.com>
54678c4c75