Dear T13 Reflector Member:
A the last meeting I took the task to update the definition of AU
(Allocation Unit) to make it easier to understand. The AU is only
used in the Configure Stream command. The term was developed
to describe the chunks of data in an AV file system that are contiguous.
There is probably no need for an AU definition since it is only
used in Configure Stream. The idea was to allow a drive to read ahead
one or more AU's and not fill the entire read buffer with data for one
of the streams. That way the read ahead buffer would be more
efficiently shared between streams.
I would like to propose that the definition of AU be dropped and the
actual use of the field be defined in the Configure Stream command.
Currently:
3.1.6 AU (Allocation Unit): The minimum number of logically contiguous
sectors on the media as used in the Streaming feature set. An Allocation Unit
may be accessed with one or more requests.
Proposed Change:
Delete AU Definition
In the Streaming Feature Set Description
Add:
4.20.2.4 Read Ahead Buffer Size
The Read Ahead Buffer Size in the CONFIGURE STREAM command (see 7.9.3.5)
indicates the recommended amount of read ahead data to buffer for the stream. This
information helps the device with pre-fetching decisions.
In Configure Stream Command (51h) change:
Word 01h, Count, Read Ahead Buffer Size
7.9.3.5 Read Ahead Buffer Size
Recommended Read Ahead Buffer Size for the stream, in
number of logical sectors. The NS (Non Sequential) Bit in READ
STREAM commands takes precedent over the Read Ahead
Buffer Size (see 7.36.3.4). Any read of the device media or
internal device buffer management as a result of the Read Ahead Buffer
Size setting is vendor specific.
Best Regards,
Daniel J. Colegrove
Hitachi Global Storage Technologies
[EMAIL PROTECTED]
(702) 614-6119
