VMS BACKUP generates two types of specially formatted data-blocks. The first type contains pure-data and consists of:
The second type of data-block contains redundancy group data. These data blocks are used for readback error CORRECTION.
A VMS BACKUP PURE DATA BLOCK
+--------------------------------------------------+
|seq#|data|data|data|data|data|data|data|......|crc|
+--------------------------------------------------+
A VMS BACKUP REDUNDANCY DATA BLOCK
+--------------------------------------------------+
|redundancy data for last group of user data-blocks|
+--------------------------------------------------+
The size of each data-block is determined by the /BLOCK_SIZE qualifier. The use of large blocks can greatly speed up backup operations and allow more data to be put onto a single reel of tape. However, when using low-density tape devices, care must be taken.
Increasing the block size speeds up data written to tape and increases disk performance because a larger block of data can be read from disk.
VMS BACKUP will use from three to five memory buffers to do its work. Each buffer contains one data-block. A 9-track reel of tape has only 10 feet of writable surface past the end-of-tape reflector. Therefore, when BACKUP detects the EOT reflector, it must write all remaining buffers to tape within that 10 feet.
If a /BLOCK_SIZE=40960 is used with a low-density tape device (1600
BPI), 25 inches of tape are consumed each time a data-block is
written to tape (40960/1600 = 25). If five buffers are being used
by BACKUP, over 10 feet of tape will be consumed when the EOT
reflector is encountered. This could cause the tape to spin off the
end of the reel. If when writing one of the blocks, BACKUP gets a
tape error, it will try to rewrite the block to tape. If the tape
is already positioned past the EOT reflector, the tape may spin off
the end of the reel trying to complete the write operation.
3.3 Sequence Number Usage
Each data-block is written with a BACKUP supplied sequence number.
If the tape drive detects a write error, then BACKUP rewrites the
data-block using the same sequence number. The sequence number is
incremented only after a "good" block has been written. When
restoring, data-blocks are only written when the sequence number
changes. In this way, only the "good" data-blocks are restored.
3.4 Error Detection and Correction
By default, VMS BACKUP does both error DETECTION and error
CORRECTION. This is because VMS BACKUP must work on a variety
of tape and disk devices and yet still maintain data integrity.
3.4.1 Cyclic Redundancy Check
For error DETECTION, BACKUP calculates a CRC (cyclic redundancy
check) value, or "checksum" on each data-block. When restoring a
VMS save-set, a CRC for the data-block is recalculated and compared
with the CRC stored within the data-block. If the two values do not
match, BACKUP knows that the data-block is bad. The /CRC qualifier
controls the use of this feature. /NOCRC turns off the feature.
3.4.2 Redundancy Group Data
For error CORRECTION, BACKUP uses redundancy group data. This data is used to reconstruct bad data-blocks being restored. A setting of ten (the default) means that if any one data-block in a group of ten data-blocks restored is bad, the one data-block can be reconstructed using the redundancy group data from the surrounding nine data-blocks. The default setting of ten causes BACKUP to devote over 17% of each data-block to redundancy group data. The /GROUP=nn qualifier controls the use of this feature. /GROUP=0 turns off the feature.
The following are performance-based suggestions. These suggestions have been tried successfully at Touch Technologies. However, TTI makes no representations or warranties regarding the effectiveness or risks associated with the use of these commands provided by Digital Equipment Corporation.
When using VMS BACKUP, the following BACKUP qualifiers are suggested:
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 tape device that does hardware-level ECC (error correction code), use the /GROUP=0 qualifier. This will reduce the extra overhead of having both BACKUP and the tape device calculating and storing ECC data to the tape. (TK50/70s, and 6250 bpi drives all have hardware-level ECC)
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).
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.
The following tape devices do their own hardware-level error detection (CRC) and error correction (ECC):
For these hardware-level CRC/ECC devices, both /NOCRC and /GROUP=0 should be used. The /NOCRC because the tape device is already generating CRC data and writing it to the tape. The /GROUP=0 because the tape device is already generating ECC redundancy data and writing it to the tape. A typical BACKUP command might be:
$ backup/rewind/image dua1: -
mub0:backup/save/BLOCK=40960/NOCRC/GROUP=0
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:backup/save/BLOCK=16384/NOCRC
When VMS has to go to a device to satisfy an I/O request.
When an I/O request can be satisfied from a data cache. No physical I/O is needed.
VMS is a virtual memory system. Programs can be run that are bigger (have more address space) than actual physical memory available.
When a process requests to read or write data.
Contains copies of data recently used by an application.
A file with a high level of activity (high I/O count).
Consists of a single 1 or 0.
One character. One byte = 8 bits (1's and 0's). "DAN" = 3 bytes.
A block is 512 bytes.
Approximately one thousand (1,024 ) bytes.
Approximately one million (1,024,000) bytes.
Approximately one billion (1,024,000,000) bytes.
Data is duplicated on two disks to provide redundancy in case of a disk failure. Also speeds up I/O operations.
Uses free memory for high-speed data caching.
RMS can set aside the most commonly used blocks of a file in a memory buffer (cache). These blocks are available only to the process that has them cached. Reads from memory are faster than reading from disk. As the number of RMS local buffers is increased, more I/O requests can be satisfied from this local buffer cache.
RMS local buffers are not shared among users.
The same as local buffering except that all processes accessing the globally buffered file have access to the cached blocks. (Global data buffers are shared among processes.) Speeds up file reads. As the number of RMS global buffers are increased, more I/O requests can be satisfied from this global buffer cache.
Physically adjacent; all in one piece.
Files that are not physically contiguous.
Making files contiguous.
A process that causes all the files on the disk to become physically contiguous. Speeds up reads and writes.
As RMS based files are written to, they become internally fragmented and disorganized. Over time, both read and write operations cause extra physical I/O operations to the RMS file. The Digital-provided CONVERT utility is used to defragment and reorganize RMS files.
Software that acts as an interpreter between the operating system (software) and a controller (firmware).
Firmware that acts as an interpreter between a device driver (software) and a device (hardware).
Software that acts as an interpreter between the operating system and another device driver.
There are nine tracks of information written to the tape at a time.
Consists of the tape controller (firmware) and the tape drive (hardware).
Tape drive that holds an 8mm tape. Can store 2.3 gigabytes (billion bytes) of data.
Synonyms people use to refer to Exabyte 8mm Tape Drive.
Amount of data that can be stored per inch of tape. Measured in BPI (Bits Per Inch), per Track.
Group Coded Recording. 6250 BPI 9-track tape drives use GCR.
Phase Encoded. 1600 BPI 9-track tape drives use PE.
1,024 bytes per second (KB/sec).
Inter-Record Gap. Gap between blocks of data written to tape.
The tape physically stops at the end of a block of data for inter-record gaps, then starts again.
The tape is kept continuously moving as long as there is enough data. If there is NOT enough data, the tape has to stop.
Error Correction Code has two purposes:
There are two kinds of error detection:
A parity check detects errors.
Each bit is read from the tape. If the parity is incorrect, there is a parity error.
Horizontal parity on each data block.
Cyclic Redundancy Code (same as CHECKSUM). Used in ERROR DETECTION.
Data is moved from the immovable disk onto removable tapes and is stored off-site for safe keeping.
Backs up the entire disk. Copies all files unless they are flagged not to be backed up.
Backs up only those files that have been modified/created since the last time that backup was run.
Restore information - Transfer data from a tape back onto the disk. (This is the real reason that tape backups are made).
Backup/Restore reads the file headers. In the file headers the qualifiers backup used when writing the saveset are stored. Backup/Restore then uses the same qualifiers when writing from tape to disk. Backup/Restore ignores any qualifiers in the Backup/Restore command.