Oracle Secure Backup Installation And Configuration Manual page 16

Oracle Secure Backup Concepts
A magnetic cassette or tape is sequential-access storage. It has a beginning and an end,
which means that to access data in the middle of the tape, a tape device must read
through the beginning part of the tape until it locates the desired data.
In a typical format, a tape drive writes data to a tape in blocks. The tape drive writes
each block in a single operation, leaving gaps between the blocks. The tape runs
continuously during the write operation.
The block size of a block of data is the size of the block in bytes as it was written to
tape. All blocks read or written during a given backup or restore operation have the
same block size. The
512-byte records contained in the block. For example, the Oracle Secure Backup
default blocking factor (128) results in a tape block size of 128*512 bytes or 64 KB.
The maximum blocking factor is an upper limit on the blocking factor that Oracle
Secure Backup uses. This limit comes into play particularly during restores, when
Oracle Secure Backup must pick an initial block size to use without knowing the actual
block size on the tape. The maximum blocking factor limits this initial block size to a
value that is acceptable to both the tape device and the underlying operating system.
When Oracle Secure Backup starts a backup, it decides what block size to use based on
several factors. Listed in order of precedence, these factors are:
Blocking factor specified using the obtar -b option
■
This option can also be specified as part of the operations/backupoptions
policy. If this option is specified, then it overrides all other factors.
Configuration of the tape drive to be used
■
You can specify what blocking factor, maximum blocking factor, or both that
Oracle Secure Backup should use for a particular tape drive when you configure
that drive. You might want to do this if you have tape drives with very different
block size limits.
Domain-wide blocking factors or maximum blocking factors set with the
■
media/blockingfactor and media/maxblockingfactor policies.
The default blocking factor (128) and maximum blocking factor (128), resulting in
■
a block size of 64K
When a blocking factor has been nominated by one or another of these factors, it must
pass the following tests:
The block size must be less than or equal to the maximum block size (blocking
■
factor) put in effect by whatever policies or tape drive configuration attributes are
in force.
The block size must be supported by the tape drive and attach point in question.
■
Sometimes a tape drive, device driver, or kernel operating system has a limitation
that supersedes all other considerations.
1-6 Oracle Secure Backup Installation and Configuration Guide
blocking factor
Oracle Secure Backup Reference for more information on the
See Also:
obtar -b option and the operations/backupoptions policy
"Configuring a Tape Drive"
See Also:
See Also: Oracle Secure Backup Reference for more information on the
media/blockingfactor and media/maxblockingfactor
policies
of a block of data expresses the number of
on page 5-17