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From: norsk@sequent.UUCP (Doug Thompson)
Newsgroups: comp.periphs,comp.sys.sgi,comp.periphs.scsi
Subject: Re: exabyte record size limit
Message-ID: <52691@sequent.UUCP>
Date: 8 Feb 91 23:14:24 GMT
References: <1991Feb8.154343.11054@helios.physics.utoronto.ca>
Reply-To: norsk@eng3.UUCP (Doug Thompson)
Organization: Sequent Computer Systems, Inc.
Lines: 34

In article <1991Feb8.154343.11054@helios.physics.utoronto.ca> sysmark@physics.utoronto.ca (Mark Bartelt) writes:
>I have an exabyte drive (from Dilog) on a 4D25.  When I recently upgraded
>to IRIX 3.3.1, I was pleased to find the /dev/{r}mt/tps*v devices:  It's
>nice to finally be able to read/write arbitrary-length tape records!
>
>However, I've found that there's a record size limit of 240k bytes.  Any
>attempts to write records longer than that return EINVAL.  Is this just a
>misfeature of the IRIX tape driver, or is it a characteristic of exabyte
>drives, or a limit (general, or SGI's) on the size of SCSI transfers, or
>what?  And, whichever, why was 240k chosen?

Its a limit of the drive itself. It has a maximum buffer (for data)
of 240kb. Cost probably was the reason. The 8500 exabyte drive has
a bigger buffer.
>
>For that matter, should I even care?  Back in ancient times, when all we
>had were half-inch drives at, say, 800 or 1600 bpi, it was worth writing
>data in blocks as large as possible, to minimize the amount of tape that
>was wasted in inter-record gaps.  How, exactly, do exabytes separate the
>physical records?  And how much tape does that information use, compared

Physical records on 1kb records grouped 8 records per helical track,
8kb per track. If data is a multiple of 8K and the device keeps streaming
no interrecord gap is present. If the device enters a stop/start mode,
then one will get gaps.

>with the length of an N-byte record?


-- 
Douglas Thompson		UUCP: ..{tektronix,ogicse,uunet}!sequent!norsk
				Internet:	norsk@sequent.com
"The scientist builds to learn; the engineer learns in order to build."  
Fred Brooks