7.1 Effects of DTA on OpenVMS BACKUP

Thus, data written by a pre-accelerated or accelerated device can be read by a device not equipped with DTA.

7.2 Recommended OpenVMS BACKUP Qualifiers

• If you have a tape device that does hardware-level error detection (check with the manufacturer... most do), use the /NOCRC qualifier. This will eliminate the extra overhead of both BACKUP and the tape device calculating CRC values.
• If you have a high-density tape device (6250 bpi or more), use the /BLOCK=40960 qualifier. This will reduce the stop/start processing that is required for inter-record gap creation. The number 40960 is a multiple of both 512 (a TK50/70 blocksize) and 8192 (blocking used by helical scan tape devices).
  A blocksize of 40960 was chosen because:
  • there is a diminishing return on using larger numbers
  • it is an easy number to remember
  • Megatape and Exabyte drives work best with a blocksize that is divisible by 8192.

  Note Some tape devices made by some vendors cannot handle high rates of speed with large /BLOCK_SIZE settings. Please consult your hardware vendor to ensure that your tape device can handle large /BLOCK_SIZE settings.

  
  
• If you have a low-density tape device (1600 bpi or less), use the /BLOCK=16384 qualifier. This will reduce the stop/start processing that is required for inter-record gap creation.
  

  Note If you are already using a blocksize greater than 16384, you do NOT NEED TO REDUCE YOUR BLOCKSIZE.

  
  

The following tape devices do their own hardware-level error detection (CRC) and error correction (ECC):

• any 6250 bpi tape drive (set at 6250 bpi)
• TK50/70s
• 4mm and 8mm high-density tape devices
• DLT tape devices
• TA90 IBM cartridge device

For these hardware-level CRC/ECC devices, /NOCRC should be used. The /NOCRC should be used because the tape device is already generating CRC data and writing it to the tape. A typical BACKUP command might be:

$ backup/rewind/image dua1: - mub0:myback/save/BLOCK=40960/NOCRC

7.2.2 Tape Devices with Minimal Error Correction

Low-density tape devices (800/1600 bpi) perform error detection, but only minimal error correction. For these devices a typical BACKUP command might be:

$ backup/rewind/image dua1: - msa0:myback/save/BLOCK=16384/NOCRC

To perform an orderly shutdown of DTA, you should log on to SYSTEM and then execute the DTA shutdown procedure.

USERNAME: SYSTEM PASSWORD: (the system password) $ @TTI_DTA:DTA_SHUTDOWN . . All DTA operations will be shutdown. . $ LOGOUT

The following is an example of the installation of DTA on a VAX or Alpha computer system using tape media:

Example A-1 Installing Dynamic Tape Accelerator

Username: SYSTEM Password: (the system password) $ CREATE/DIRECTORY SYS$SYSDEVICE:[TTI_DTA] $ SET DEFAULT SYS$SYSDEVICE:[TTI_DTA] $ DIRECTORY No files found. $ ALLOCATE MSA0: _MSA0: allocated $ MOUNT MSA0:/FOREIGN Volume is write locked DTA mounted on _MSA0: $ BACKUP/LOG MSA0:DTA *.*/NEW_VERSION . . . $ DISMOUNT MSA0: $ DEALLOCATE MSA0: $ @DTA_STARTUP Dynamic Tape Accelerator - Startup Procedure Copyright (c) 1992, 1997 Touch Technologies, Inc. DTA_STARTUP V4.0 Copyright 1992, 1997 Touch Technologies, Inc. All Rights Reserved. $ @CONTROL ACCEL MSA0: 40 Dynamic Tape Accelerator - Tape Acceleration Procedure Copyright (c) 1988, 1997 Touch Technologies, Inc. DTA_CONTROL V4.0 Copyright 1992, 1997 Touch Technologies, Inc. All Rights Reserved. MSA0: accelerated $ log SYSTEM logged out at 21-JAN-1997 09:50:59.95

The following is an example of changing SYSGEN parameters to meet or exceed those values given in Chapter 2. In order to change SYSGEN parameters for Dynamic Tape Accelerator, the OpenVMS SYSGEN facility must be run. Since NON-DYNAMIC parameters are affected, the USE CURRENT command must be specified. Each parameter must then be shown. If the parameter is lower than the required value, it must be changed to the required value.

The SYSGEN parameter changes can be made either directly through the OpenVMS SYSGEN utility, or by adding the following lines to your SYS$SYSTEM:MODPARAMS.DAT file and running AUTOGEN with REBOOT.

ADD_NPAGEDYN = 204800 ! change for DTA ADD_SPTREQ = 38000 ! change for DTA

Example B-1 Changing SYSGEN Parameters

APPLICATIONS SYSTEM VAX/OPENVMS Version 5.5 Username: SYSTEM Password: (the system password) $ SET DEFAULT SYS$SYSTEM $ RUN SYSGEN SYSGEN> USE CURRENT SYSGEN> SHOW SPTREQ Parameter Name Current Default Minimum Maximum SPTREQ 4000 2500 -1 -1 SYSGEN> SET SPTREQ 42000 . . SHOW and SET as necessary for each required parameter . SYSGEN> WRITE CURRENT

The DTA distribution directory contains less than 500 blocks of disk space. The distribution directory includes all of the files required for VAX OpenVMS 5.5 and greater or Alpha OpenVMS 6.1 and greater. When the DTA_STARTUP procedure is executed, it selects the files needed for your version of OpenVMS. In addition, various files helpful to our support people are included in the directory.

Dynamic Tape Accelerator uses 10% more CPU time than normal tape I/O operations. The actual CPU time varies based on the type of tape I/O requests being processed, and the type of CPU that DTA is running on.

When using DTA data compression, up to 70% more CPU time than normal can be used by DTA. Dynamic Tape Accelerator data compression requires about 10 VUPs for every 400 KB/sec of data being compressed. For example, using DTA data compression on a 800 KB/sec tape device would require a 20 VUPs CPU system to run the tape device at full speed.

The DTA device driver uses less than 204800 bytes of non-paged dynamic memory (NPAGEDYN). This memory is used for both code and internal data structures.

Dynamic Tape Accelerator uses the system logical TTI_DTA to store the location of the DTA package. This logical is created by the DTA_STARTUP.COM command file.

Dynamic Tape Accelerator DTA_STARTUP command, and the CONTROL command which uses the DTA_CONTROL.EXE image and has the options, PREACCEL, ACCEL, DEACCEL, require the following privileges:

• CMKRNL
• OPER
• PHY_IO

The MONITOR procedure is used to monitor Dynamic Tape Accelerator's effect on data transfer rates to tape. The MONITOR procedure can be executed when you have executed the PREACCEL or ACCEL command. In order to run the monitor, you must have the following privileges:

• CMKRNL
• OPER
• WORLD

This is a partial list of tape devices. Included is the maximum data transfer rate for each drive. Use this list to compare normal data transfer rates with DTA accelerated transfer rates.

Table E-1 Maximum Tape Drive Data Transfer Rates

Drive	Recording Density (bpi)	Tape Speed (ips)	Max. Data Transfer Rate
TSV05	1600	45	72 KB/sec
TE16	800/1600	45	36/72 KB/sec
TU45	800/1600	75	60/120 KB/sec
TU77	800/1600	125	100/200 KB/sec
TU78	1600/6250	125	200/781 KB/sec
TU80	1600	25/100	160 KB/sec
TU81	1600/6250	25/75	120/468 KB/sec
TU81+	1600/6250	25/75	120/468 KB/sec
TA81	1600/6250	25/75	120/468 KB/sec
TA78	1600/6250	125	200/781 KB/sec
TU79	1600/6250	125	769 KB/sec
TF30	6666	75	45 KB/sec
TK50	6666	75	45 KB/sec
TK70	10000	100	90 KB/sec
TA90	38000		2000 KB/sec
TF85	19200		800 KB/sec
TF86	19200		800 KB/sec
4MM	19200		183 KB/sec
8MM	19200		246 KB/sec

All DEC and DEC-compatible 9-track magtape drives set at a density of 1600 bpi comply with the ANSI X3.29 1600 PE (phase encoded) standard. The standard requires the following methods of error detection and error correction to be done by the tape hardware:

• Error detection:
  • vertical parity checking (each byte)
  • longitudinal parity checking (each tape block)
  • read-after-write check
  • detects odd-bit errors and double-bit errors
• Error correction:
  • read-after-write, rewrite if error
  • read-parity-check, reread if error
  • corrects single bit errors

All DEC and DEC-compatible 9-track magtape drives set at a density of 6250 bpi comply with the ANSI x3.54 6250 GCR (group coded recording) standard. GCR organizes each seven data bytes into a group.

The standard requires the following methods of error detection and error correction to be done by the tape hardware:

• Error detection:
  • vertical parity checking (each byte)
  • longitudinal parity checking (each tape block)
  • read-after-write check
  • odd-bit error and double-bit error detection
  • double-bit error detection per group
  • CRC on each tape block
  • parity check on each CRC
  • multiple copies of each CRC
  • parity check on each ECC character
• Error correction:
  • read-after-write, rewrite if error
  • read-parity-check, reread if error
  • correct single-bit errors per byte
  • correct single-byte errors per group
  • one ECC byte for each seven data bytes per group
  • ECC character for each group of CRC characters

DEC TK50 and TK70 tape systems organize data into groups of 512 bytes. Following every eight groups are two ECC groups. One ECC group for every four data-groups. The tape systems use the following error detection and correction schemes:

• Error detection:
  • CRC on each group
  • read-after-write check, rewrite if error
  • format field check when reading
  • specialized retry circuits of a write or read error
• Error correction:
  • read-CRC-check, reread or correct if error
  • correct any error within a single block for every four blocks