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From: Daniel Phillips <phillips@bonn-fries.net>
To: "Ken Hirsch" <kenhirsch@myself.com>, "Chris Wedgwood" <cw@f00f.org>,
        "John Alvord" <jalvo@mbay.net>
Subject: Re: [PATCH] 64 bit scsi read/write
Date: Mon, 16 Jul 2001 00:14:21 +0200
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On Sunday 15 July 2001 15:16, Ken Hirsch wrote:
> Chris Wedgwood <cw@f00f.org> wrote:
> > On Sat, Jul 14, 2001 at 11:05:36PM -0700, John Alvord wrote:
> > >
> > >     In the IBM solution to this (1977-78, VM/CMS) the critical data
> > > was written at the begining and the end of the block. If the two
> > > data items didn't match then the block was rejected.
> >
> > Neat.
> >
> > Simple and effective.  Presumably you can also checksum the block,
> > and check that.
>
> The first technique is not sufficient with modern disk controllers,
> which may reorder sector writes within a block.  A checksum,
> especially a robust CRC32, is sufficient, but rather expensive.

As somebody else pointed out, not if you don't have to compute it on
every block, as with journalling or atomic commit.

> Mohan has a clever technique that is computationally trivial and only
> uses one bit per sector:
> http://www.almaden.ibm.com/u/mohan/ICDE95.pdf
>
> Unfortunately, it's also patented:
> http://www.delphion.com/details?pn=US05418940__

Fortunately, it's clunky and unappealing compared to the simple 
checksum method, applied only to those blocks that define consistency
points.  I don't think this is patented.  I'd be disturbed if it was,
since it's obvious.

> Perhaps IBM will clarify their position with respect to free software
> and patents in the upcoming conference.

Wouldn't that be nice.  Imagine, IBM comes out and says, we admit it,
patents are a net burden on everybody, even us - from now on, we use
them only against those who use them against us, and we'll put that
in writing.  Right.

--
Daniel
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