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Oracle Secure Backup Concepts

1-6

Oracle Secure Backup Installation and Configuration Guide

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

blocking factor

of a block of data expresses the number of

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.

See Also:

Oracle Secure Backup Reference

for more information on the

obtar

-b

option and the

operations/backupoptions

policy

See Also: