XOR Command meeting minutes 1/9/95 -- Document number X3T10/95-114r0

Jay Elrod Jay_Elrod at notes.seagate.com
Wed Jan 11 10:48:40 PST 1995

XOR Command Study Group meeting minutes - Document number X3T10/95-114r0
Meeting place: Harrah's, Lake Tahoe
Date: January 9, 1995

Gerry Houlder  Seagate
Jay Elrod  Seagate
Bill Hutchison  Hewlett Packard
Doug Prins  Q-Logic
Larry Lamers  Adaptec
Paul Hodges  IBM
Keith Staub  Quantum
Paul Peterson  Conner
James McGrath  Quantum
John Baudrexl  Fujitsu/Intellistor
George Penokie IBM
Bob Snively  Sun Microsystems
Paul Boulay  Hitachi Computer Products
Thai Nguyen  StorageTek
Duncan Penman IIX
Giles Frazier  IBM Austin
Jeanne Martin  LLNL
Lansing Sloan  LLNL
Gene Milligan  Seagate

Gerry Houlder acted as chairman for the meeting.  The issues discussed are
summarized below.

1) Newly added 10 byte XDWRITE command - the NDisc bit will be moved from
bit 5 to bit 2 (of byte 1) to more closely resemble the 16 byte XDWRITE command.

2) RAID Group Address mode page - the concept of multiple RAID groups, LBA
ranges, etc. will be removed. (The whole mode page may be removed for the time
being, until a need is demonstrated for a more-than-one-byte physical secondary

3) Opcodes for XOR commands were presented, with no objection. The six new
opcodes are:

 80h = XDWRITE (16)
 81h = REBUILD
 50h = XDWRITE (10)
 51h = XPWRITE
 52h = XDREAD

4) The NDisc bit will be put back into the 16 byte XDWRITE command for those who
wish to use the XDWRITE command (rather than REGENERATE) during a controller
supervised rebuild or regenerate operation. (A "controller supervised" rebuild 
regenerate is one where the devices do not have peer to peer capability, and the
controller must send XOR commands to all devices involved.) The NDisc bit will
occupy bit 2 of byte 1.

5) Once again, the issue of whether to consolidate the MAX XDWRITE SIZE and
MAX XPWRITE SIZE fields in the RAID Control mode page was discussed. The
two fields will be consolidated into one field which will occupy bytes 4 - 7 of 
RAID Control mode page. Bytes 8 - 11 will be reserved.

6) It was recommended that there be information available in the RAID Control 
page regarding total buffer space available for XOR type operations. No final 
was reached as to how this would best be done. It was agreed, however, that 
should be a "buffer full" status byte to indicate when there is no more room 
in the buffer. This buffer full condition results (for example) from XOR data 
sitting in the
buffer waiting for an XDREAD command. A "buffer full" status byte will be 
proposed to
the appropriate committee.

7) There was discussion regarding how long XOR data should remain in the buffer
of an XOR device following an XOR command which would be expected to be followed
by an XDREAD command (10 byte XDWRITE, for example). This issue will be covered
in the next release of the XOR document, stating the events which can cause the 
data to be cleared.

8) It was recommended that an implementor's note be added to any commands which
require a follow up XDREAD command, stating that linking of the XDREAD command
to the associated prior command is recommended for data integrity.

9) It was recommended that the models at the front of the XOR document be 
to include more examples of possible RAID configurations, along with recommended
procedures for executing RAID functions in those configurations.

10) For reasons of SPC compatibility it was recommended that the 16 byte XDWRITE
command be restructured. As a result of this discussion, the Secondary Address 
will be moved from bytes 12-14 to byte 14 (reducing it from 3 bytes down to 1 
byte), and
the Transfer Length field will be expanded from bytes 10-11 to bytes 10-13.

11) For reasons of compatibility with the REBUILD and REGENERATE commands, the
Secondary Control field of the 16 byte XDWRITE command will be moved to bits 
0-1 of
byte 1.

12) It was recommended that an "XOR Disable" bit be added to the RAID Control 
page to allow the controller to disable any XOR functionality within the 
device. The
thinking is that in some designs this may free up any buffer space which may 
otherwise been designated for use in XOR operations. Bit 1 of byte 2 of the 
RAID Control
mode page will be used for the XOR Disable bit.

13) It was recommended that the Mirror bit be removed from the 16 byte XDWRITE
command since the operation of the command is radically altered by this bit to 
the point
that no XOR functions are performed. This bit will be removed in the next 

14) It was recommended that an XOR bit be added to Inquiry data, indicating XOR
functionality in the device. This will be proposed to the appropriate committee.

15) It was recommended that the statement on page 22, regarding how far an 
command shall have progressed before sending good ending status when write 
is enabled, be removed. This will be done.

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