Comment on XOR Meeting Minutes

Shahe Krakirian x2130 shahek at
Wed Nov 23 17:35:36 PST 1994

In the XOR Command meeting minutes 11/7/94, Jay Elrod wrote:

> 7) The question was raised by Steve Fuld as to whether there needs to be
> entries for both the Max XPWRITE and Max XDWRITE lengths in the RAID Control
> mode page. There was no good case made during the meeting for leaving both of
> these fields in the mode page. Jay Elrod will investigate this further and
> consolidate the two fields into one field if no reason can be found to keep
> them both.

I can see one reason for keeping the Max XDWRITE Size and Max XPWRITE size 
different. During an XDWRITE command with NDisk=0, both the write data and the 
XOR data have to be held in the drive's buffer simultaneously (XOR data is 
generated and stored in the buffer before the write data in the buffer is
written to the disk). During an XPWRITE, the XOR data can replace the write
data in the buffer as it is being generated (there is no use for the
write data once the XOR data is generated). Therefore, an XDWRITE command
can use up twice as much buffer memory compared to an XPWRITE command with
the same transfer length.

Conversely, a drive may optimize buffer memory usage by suppporting a 
Max XPWRITE Size that is twice the Max XDWRITE Size.

Shahe Krakirian


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