Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!ucbvax!hplabs!otter!hpcpbla!kev
From: kev@hpcpbla.HP.COM (Kevin Jones)
Newsgroups: comp.sys.hp
Subject: Re: tcio(1) read-while-write question
Message-ID: <660007@hpcpbla.HP.COM>
Date: 15 Jan 90 08:25:30 GMT
References: <32870010@hpuepta.UUCP>
Organization: HP Computer Peripherals Bristol, UK
Lines: 30

The 9144A, 35401, 7914CT, 7946 drives all have read-while-write capability
in hardware. Data written by a write head is immediatly read back by 
a read head. Verification is thus achieved "on the fly" making a verify
pass redundant. Hence the "-V" option in tcio to inhibit the verify pass.

The aforementioned drives will perform read-while-write irrespective of the 
OS revision. If an error is detected in the written data then these drives
will perform retries. If the retries fail then the block being written 
will be re-mapped and written to a spare block on tape. 

Read-while-write, retries and sparing are all performed by the drives 
themselves. The OS (whatever its revision) has no involvement in these
processes. 

The 9145 drive also does its own read-while-write, retries and sparing.

I'm not sure what the 9142 drive does.

Note: 7914P (disk with push-button-backup) tape units do not posess read
while write ability. 


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Kevin Jones.                       | Hewlett Packard Ltd,
                                   | Computer Peripherals Bristol,
kev%hpcpbla@hplb.hpl.hp.com        | Filton Road,
                                   | Stoke Gifford,
Tel: 011 44 272 799910 (ext 22351) | Bristol.   BS12 6QZ.
                                   | ENGLAND.
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