[linux-lvm] Re: [PATCH] 64 bit scsi read/write

[Ken Hirsch]

Chris Wedgwood <cw at 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.

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__

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