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From: mcconnel@b11.ingr.com (Guy McConnell)
Newsgroups: comp.periphs,comp.sys.sgi,comp.periphs.scsi
Subject: Re: exabyte record size limit
Message-ID: <1991Feb18.175737.19866@b11.ingr.com>
Date: 18 Feb 91 17:57:37 GMT
References: <1991Feb8.154343.11054@helios.physics.utoronto.ca> <52691@sequent.UUCP> <64513@brunix.UUCP>
Reply-To: mcconnel@b11.UUCP (Guy McConnell)
Organization: Intergraph Corp. Huntsville, AL
Lines: 43

In article <64513@brunix.UUCP> sgf@cs.brown.edu (Sam Fulcomer) writes:
[In regard to Exabyte record size limits]

>Well, some people have it mostly right. The 8200 has a 256k data buffer.
>The maximum logical block length for fixed or variable block modes is 240k.
>the physical block length is 1024 bytes. The data buffer circuitry 
>adds block address and tag information in the data buffer, hence the 240k
>limit.
>
>The 8200 format writes 8 physical blocks of data (1024 bytes each, with 400 
>bytes ECC and 16 bytes address) during each physical write operation. 

    Actually,  there are 400 bytes of ECC,  14 bytes of address, and
2 bytes of CRC per 1K block.  The drive writes more than the 8 1K
blocks during each "physical write" in which a larger logical block
size was specified.  Perhaps you meant during each rotation of the
head during a write.
>
>I assume that most sensible drivers use buffered mode writes, however.
>In buffered mode writes the data buffer fills up to a predetermined
>point before the drive actually writes the data to tape. The predetermined
>point is called the "write motion threshold" and defaults to 240k.

    It is called the "motion threshold" because it is equally valid
for reads and writes and it defaults to 80H or 128KBytes.  It is
Mode Selectable and the valid values are from 20H to D0H (32KBytes
to 208KBytes).
--
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all those who
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