Date: March 22, 1991 To: X3T9.2 Membership From: John B. Lohmeyer, X3T9.2 Chairman Subject: Data Compression Working Group Meeting -- March 18, 1991 John Lohmeyer called the meeting to order at 9:00 a.m., Tuesday, January 15, 1991. He thanked Dal Allan of ENDL for hosting and arranging the meeting. As is customary, the people attending introduced themselves. A copy of the of the X3T9.2 membership list was circulated for attendance and corrections. The following people attended the meeting: Name Status Organization ------------------------------ ------ ------------------------------ Mr. Kenneth Post P Future Domain Mr. Steve Krupa V Hewlett Packard Co. Mr. Lawrence J. Lamers P Maxtor Corp. Mr. John Lohmeyer P NCR Corp. Mr. Gene Milligan A Seagate Technology Mr. Robert L. Simpson P Sony Corp. of America Mr. Dennis Appleyard P Storage Technology Corp. Mr. David Griffith O Wang Laboratories 8 People Present Status Key: P - Principal A - Alternate O - Observer V - Visitor RESULTS OF MEETING Steve Krupa brought copies of his latest proposal (90-119 R2) on defining a Data Compression mode page. This mode page is designed specifically for DDS devices, but it could be used for any other sequential-access device. This version was considerably reduced in scope from earlier versions. Steve noted that the present facility in SCSI-2 for data compression was inadequate for several reasons: 1. It only supports 7 bits to identify the compression algorithm. X3B5 has requested that ISO/IEC JTC-1 establish a registration authority to assign codes to compression algorithms. These are to be 8-bit codes. 2. The QIC Committee has defined a code scheme for quarter-inch devices using all of the vendor unique codes in SCSI-2 (80h--FFh). (This scheme further reduces the compression code space to 6 bits.) 3. The SCSI-2 facility does not include support for devices that can manage changes to the compression algorithm on their own. The SCSI-2 facility assumes the initiator manages everything. 4. SCSI-2 does not provide separate fields for compression and decompression algorithms. Since the SCSI-2 facility was deemed inadequate for SCSI-3 it was felt that the best approach to solving the problem was to define an entire new page to control data compression. While Dal Allan had requested that the page also include support for encryption/decryption, no one in the group felt competent to address this issue. It was decided that a separate page should be used if/when a specific need for encryption is identified. The group then focused on improvements to Steve's latest proposal. The compression algorithm field was expanded to 32 bits in anticipation that any registration authority will need more than 256 code values to manage. The PS bit was added to bit 7 of byte 0 to be consistent with other mode pages. The RED field was expanded to two bits to provide a third option on CHECK CONDITION reporting. The Sense Key to be reported on algorithm changes will be RECOVERED ERROR instead of NO SENSE. The code assigned to DECOMPRESSION EXCEPTION was changed to 70h. When Steve presented the results of this working group meeting to the general working group meeting later in the week, a further change was identified. There is no need for a code value of 01h to tell the target to select its default algorithm. The MODE SENSE command with report default values can accomplish the same function. The wording describing code value 01h as a special value will be deleted. The revised mode page is as follows: Table X-XX: Data Compression Characteristics Page ============================================================================== Bit| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | ============================================================================== 0 | PS |Reserved| Page Code (0Fh) | -----|-----------------------------------------------------------------------| 1 | Page Length (06h) | -----|-----------------------------------------------------------------------| 2 | DCE | Reserved | -----|-----------------------------------------------------------------------| 3 | DDE | RED | Reserved | -----|-----------------------------------------------------------------------| 4 | (MSB) | -----|--- ---| 5 | | -----|--- Compression Algorithm ---| 6 | | -----|--- ---| 7 | (LSB) | -----|-----------------------------------------------------------------------| 8 | (MSB) | -----|--- ---| 9 | | -----|--- Decompression Algorithm ---| 10 | | -----|--- ---| 11 | (LSB) | -----|-----------------------------------------------------------------------| 12 | Reserved | -----|-----------------------------------------------------------------------| 13 | Reserved | -----|-----------------------------------------------------------------------| 14 | Reserved | -----|-----------------------------------------------------------------------| 15 | Reserved | ============================================================================== Steve planned to revise his proposal in time for the May mailing.