Seagate STT20000A, Руководство по эксплуатации продукта

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Chapter 6 Tape Format
Page 66 STT20000A Product Manual
Track numbering
All even numbered tracks, and the Directory Track, are recorded in the forward
direction (the direction from the BOT marker to the EOT marker). All odd numbered
tracks are recorded in the reverse direction (the direction from the EOT marker to
the BOT marker). All even numbered tracks are located below the Directory Track;
all odd numbered tracks are above it.
Track format
Tracks are recorded sequentially beginning with track 0, then track 1, and so on.
Before recording, data are grouped into blocks, and blocks are grouped into frames
with 128 blocks per frame.
Two numbering methods are used for blocks: physical numbering and logical
numbering. Both numbers start from 0 at the beginning of each partition.
•
Physical numbering is related directly to the recorded block on the tape. Each
new block is given a unique physical number, regardless of its contents.
•
Logical numbering does not relate to the blocks physically recorded on the tape;
rather, this type of numbering is the block numbering system used by the host
computer. Often the host system operates with logical blocks that are a different
size from the 512-byte blocks that are physically recorded on the tape.
The host blocks can be larger or smaller than 512-bytes and can also be fixed
or variable. Fixed host blocks contain the same number of data bytes in each
block. Variable host blocks may contain a different number of data bytes in each
block.
The format provides both a physical block number for each block recorded on the
tape and a logical block number that can span more than one physical block. These
two numbers are recorded in the control field of every block.
Frames
Every track on the tape is recorded in blocks that contain 512 data bytes. The data
bytes are 8-bit bytes, which are numbered b0 to b7 with b7 being the most
significant bit.
A frame is made up of 128 blocks—108 data blocks plus 20 error-correction code
(ECC) blocks. Figure 6-1 illustrates the general track layout of sequentially recorded
frames.
Figure 6-1
General track layout
Frame N Frame N+1 Frame N+2 Frame N+3 Frame N+4