Streaming device questions

Burn Alting burn at
Sun Aug 22 16:58:48 PDT 2004

* From the T10 Reflector (t10 at, posted by:
* Burn Alting <burn at>
Hi Peoples,

I have a question regards variable block sizes and logical block
identifiers and applications that use stream devices in general.

I am trying to get a handle on whether or not to support variable block
sizes on a simulated tape device (it's a disk). If I accept variable
block sizes, then I am going to have to carry an overhead of the logical
block identifiers somewhere on the disk.

Do many applications in the real world demand variable block sizes?

Do such applications have a mix of variable and fixed IO's?

Consider a streaming device which can accept variable block sizes and
the following sequence of writes are made (with the fixed bit = 0. Also
tape starts at BOP).

	Write 32K, Write 32K, Write 16K, Write 16K, Write 32K

Does this mean I have 5 logical block identifiers?

What then, if I wrote (after the above five writes)

	Write 1, Write 1

with the fixed bit set to 1 and the logical block size was 32K.

Another 2 logical block identifiers?

What happens if the first five writes above occur and then one rewinds
the device to BOP and then issues a variable block sized read of 72K -
will/should it work?

Thanks in advance (and bearing with my potentially ignorant questions)


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