User Guide

3-4 Ampex 1308911-02
Model No.
Running Head
DD-2 Format Types DD-2 Tape Format
3.4.3 File Sections
A file section consists of zero or more consecutive data blocks bounded on each end by a
filemark, BOP, or EOD. File sections are numbered sequentially within a partition, starting at
one.
Some older tape formats use a set of two consecutive filemarks to indicate EOD. Because the
DD-2 format keeps track of EOD independently, double filemarks are not required; they
merely indicate an empty file.
Note: Each additional filemark advances the file section number by one.
The DD-2 format requires that each file section start at the beginning of a physical block (but
the file section can span multiple physical blocks). Thus, writing many small file sections will
waste space on the tape.
3.4.4 Data Blocks
The following paragraphs discuss data block sizes and the relationship between tape drive
physical and logical data blocks.
Physical Blocks. Physical blocks are storage locations for data written to tape. They are the
smallest units that the drive can use when writing data to tape from the buffer. Each physical
block is 1,199,840 bytes. Each file section must start in a separate physical block (but it can
span multiple physical blocks). To conserve partition space try to avoid writing small file
sections that waste significant physical block space (for example, writing ten 200 kB file
sections would consume approximately 12 MB of space on tape).
Logical Data Blocks. A data block (also called a logical data block or a record) is a series of
consecutive bytes of a fixed length. Any given file section has only one data block size. Each
data block is followed by a two-byte checksum unless you disable the checksum feature or
operate the drive in byte-stream mode.
Note: Ampex does not recommend turning off checksums or changing the default data block
size unless you want to operate in byte stream mode.
Data blocks are numbered by:
• Logical Data Block Number (LDBN) within a file section, starting at zero.
• Cumulative Data Block Number (CDBN) within a partition, starting at one (filemarks are
counted as blocks for addressing purposes). CDBN is also referred as block address.
Data Block Size. The tape drive accepts data block sizes of one byte (byte-stream mode), or an
even number of bytes between 80 and 1,199,840 (1,199,838 when using checksums). Data
blocks cannot span physical block boundaries. Therefore, best use of space is achieved by data