Streaming device questions

Banther, Michael michael.banther at hp.com
Mon Aug 23 03:45:11 PDT 2004


* From the T10 Reflector (t10 at t10.org), posted by:
* "Banther, Michael" <michael.banther at hp.com>
*
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See embedded comments below.

Regards,
Michael Banther
Hewlett-Packard Ltd.
Telephone +44 (117) 312-9503


-----Original Message-----
From: owner-t10 at t10.org [mailto:owner-t10 at t10.org] On Behalf Of Burn
Alting
Sent: 23 August 2004 00:59
To: t10 at t10.org
Subject: Streaming device questions


* From the T10 Reflector (t10 at t10.org), posted by:
* Burn Alting <burn at goldweb.com.au>
*
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?
[Michael Banther] Most backup-restore applications use them to some
extent.

Do such applications have a mix of variable and fixed IO's?
[Michael Banther] Yes.

Consider a streaming device which can accept variable block sizes and
the following sequence of writes are made (with the fixed bit =3D 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?
[Michael Banther] Yes.

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?
[Michael Banther] Yes.


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?
[Michael Banther] Depends on the setting of the SILI bit in the Read =
CDB
(see SSC3r01b, 5.3 & 6.4 and associated text).

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

Burn


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