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From: Wilko Bulte <wilko@yedi.iaf.nl>
Message-Id: <199612281749.SAA10725@yedi.iaf.nl>
Subject: Re: Q: ideal block size for exb 8505
To: joerg_wunsch@uriah.heep.sax.de
Date: Sat, 28 Dec 1996 18:49:02 +0100 (MET)
Cc: freebsd-scsi@FreeBSD.org, ping@red.stepnet.com
In-Reply-To: <199612281549.QAA12728@uriah.heep.sax.de> from "J Wunsch" at Dec 28, 96 04:49:51 pm
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As J Wunsch wrote...
> As Ping Mai wrote:
> 
> > I am using dump to backup to a exb 8505xl.  can anyone
> > recommand an ideal block size to get max through
> > put?  i understand that one can also set the block size
> > using mt.  how's this diff from blocksize given to
> > dump?  i am confused.
> 
> You can only set a fixed blocksize with mt(1).  Most drives limit the
> ability to accept a fixed blocksize to 512 bytes, some also allow for
> 1024 bytes.  Most (and all modern) drives don't work with a fixed
> block size, but with a variable one (expressed as 0).  This basically
> means they write the amount you pass to the driver in a single
> write(2) call into a single tape block.  Reading these blocks requires
> a read(2) of at least the same (or a larger) size, and will return
> exactly the amount of bytes that have been written into that block
> (i.e., will yield a `short read' when attempting to read more bytes
> than have been there in the tape block -- that's how restore(8)
> examines the tape block size).
> 
> Something like 32 KB is a good value.  Larger values reduce the
> overhead, but the impact is probably getting neglicible above
> something like 10 KB.  Some programs default to 512 bytes only which

Keep in mind that drives that use hardware compression really appreciate
a big blocksize. For example DLTs really like 32kB, 10kB is too small
(for compressible data) to keep 'm really working at max efficiency.

But again: highly data and device dependent.

Wilko
_     ____________________________________________________________________
 |   / o / /  _  Bulte  email: wilko@yedi.iaf.nl - Arnhem, The Netherlands
 |/|/ / / /( (_) 	Do, or do not. There is no 'try' - Yoda
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