[BLOCK] I/O barrier documentation update

The generic i/o scheduler would make sure that it places the barrier request and

all other requests coming after it after all the previous requests in the

queue. Barriers may be implemented in different ways depending on the

driver. A SCSI driver for example could make use of ordered tags to

preserve the necessary ordering with a lower impact on throughput. For IDE

this might be two sync cache flush: a pre and post flush when encountering

a barrier write.

There is a provision for queues to indicate what kind of barriers they

can provide. This is as of yet unmerged, details will be added here once it

is in the kernel.

driver. For more details regarding I/O barriers, please read barrier.txt

in this directory.

1.2.2 Request Priority/Latency