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Date:      Thu, 22 Apr 1999 12:01:12 -0700 (PWT)
From:      Matthew Jacob <mjacob@feral.com>
To:        Joerg Wunsch <joerg_wunsch@uriah.heep.sax.de>
Cc:        freebsd-scsi@FreeBSD.ORG
Subject:   Re: i/o error with larger QIC
Message-ID:  <Pine.LNX.4.04.9904221156160.317-100000@feral.com>
In-Reply-To: <19990422203034.22930@uriah.heep.sax.de>

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> 
> write			read speed using blocksize
> blocksz	speed		f/512	f/1024	v/10K	v/32K
> 				**
> 
> v/32K	179K/s		+	+	*	188K/s
> v/10K	179K/s		+	+	190K/s	(190K/s)
> v/1024	104K/s		+	137K/s	(81K/s)	(81K/s)
> f/1024	179K/s **	++	133K/s	83K/s	82K/s
> f/512	179K/s		87K/s	121K/s	191K/s	192K/s
> 
> () Values in parenthesis are modi that are possible but not much
>    useful, since they return short blocks.
> 
> +  IO error
> 
> ++ IO error plus ``Invalid request.  Fixed block device requests must
>    be a multiple of 1 bytes'' (apparently some bug there)

D'oh! That's a wierd one! 


> 
> *  not useful, since only partial blocks would be retrieved, so
>    the remaining data in each block were lost; IMHO this should rather
>    yield an IO error instead (which it did in previous FreeBSD versions)
> 
> ** blocksize currently needs to be manually adjusted back to 1024
>    after each tape operation, since the driver resets it to 512

That's a bug to fix, yes.

> 
> To summarize:
> -------------
> 
> Variable-length logical blocks seem to work best, ever.


You see- this is why this all has come to be a mess. *My* out of date
experience with QIC had led me to believe that Variable && QIC was just
not a good idea. I'm still not convinced of this (see other mail about
what may be a design flaw)- but this is why I'm actually getting a QIC
drive so I can assess, for myself (not out of manuals), what's going on.



> Fixed-legnth blocks (either 512 or 1024 bytes) have problems with read
> speed.  This seems to be a driver problem, in particular since the
> read speed using variable-length read operations is better.  Given
> that the best possible values are at best en par with the variable-
> length recording values, i don't see why anybody would use fixed-
> length mode.

> 
> The driver seems to have bugs, see ++ and ** above.  Btw., `mt stat'
> reports compression 0x3 everywhere, although i know that only QIC-2GB
> and above support compression at all.  Maybe this is part of the
> problem?
> 
> Slow speeds above are accompanied by the tape going in `start-stop'
> mode.
> 

A more complete assessment than this will be performed soon. I don't think
fixed mode is that great either, but I'm not convinced that variable mode
with QIC drives is so fanatastic either.

-matt




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