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Help on Tape LBA



 
 
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  #1  
Old November 4th 03, 12:35 PM
Shiera
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Default Help on Tape LBA

Hello everyone...

Just some follow-up questions on Malc's answers to my previous
questions...

Since data are stored sequentially, so the block positioning are also
sequential, right? How can we identify an end of the block, especially
if a single cartridge contains blocks of different sizes? Is this
where filemarks come in?

Once again, thank you so much for your help. ^-^

- shiera -
  #2  
Old November 5th 03, 01:42 AM
Malcolm Weir
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Default

On 4 Nov 2003 03:35:08 -0800, (Shiera) wrote:

Just some follow-up questions on Malc's answers to my previous
questions...

Since data are stored sequentially, so the block positioning are also
sequential, right?


Yep.

How can we identify an end of the block, especially
if a single cartridge contains blocks of different sizes? Is this
where filemarks come in?


I suspect a lot of your comments come from not realizing that what the
application sends to the drive is not the same bit pattern as is
recorded on tape.

For example, if you are using a drive with variable-length block, you
may send the following to a drive:

B1 BBBB2 BB3 BBBBBBBB4 B5

(Where the length of the each block is indicated by the number of
"B"s).

The recorder may actually write something like this:

Start_of_Recording
Start_Block
Block_Length=2
Data_1
End_Block
CRC_Block1
Start_Block
Block_Length=5
Data_2
End_Block
CRC_Block2
Start_Block
Block_Length=3
Data_3
End_Block
CRC_Block3
Start_Block
Block_Length=8 (the max)
Data_4
Incomplete_Block
CRC_Block4A
Start_Continuation_Block
Block_Length=1
End_Block
CRC_Block4B
Start_Block
Block_Length=2
Data_5
End_Block
CRC_Block5
End_of_File
End_of_Recording

(This is an entirely hypothetical example, and doesn't represent any
recording format I know, but is suggestive of a number of techniques).

As you may have noticed, I am treating Block4 as being too big to be
stored as a single entity on tape; this may be a result of a limited
number of bits for the "Block_Length" or perhaps a desire to limit the
amount of data covered by the CRC. So on the fictional tape it is
split into two separate entities, with special markings to show that
it is a split block.

It is clear, though, that the tape format I illustrated can reproduce
the data pattern sent to it.

Obviously, to position to an arbitary block "n", you just skip "n-1"
"End_Block" indicators...

Now, that particular fictional tape format is actually pretty useless,
because of what would happen if a tape had a bad spot incapable of
recording good data? If the bad spot obligingly occurred in the
middle of the user data, you'd spot the problem as soon as you read
the CRC, but if the unrecordable area covers the "Block_Length" field
you are in trouble!

So real tape formats include yet more data so that even if the bad
spots occur, you can simply repeat the troublesome block and, on
playback, recognize that the previous copy of a given block should be
discarded in favor of the second (or third, or whatever) attempt at
writing it!

- shiera -


Malc.
 




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