On 28.12.2011 11:22, Simon Riggs wrote:
On Wed, Dec 28, 2011 at 7:42 AM, Heikki Linnakangas
<heikki.linnakan...@enterprisedb.com>  wrote:

How would you know when to look in the double write buffer?

You scan the double-write buffer, and every page in the double write buffer
that has a valid checksum, you copy to the main storage. There's no need to
check validity of pages in the main storage.

OK, then we are talking at cross purposes. Double write buffers, in
the way you explain them allow us to remove full page writes. They
clearly don't do anything to check page validity on read. Torn pages
are not the only fault we wish to correct against... and the double
writes idea is orthogonal to the idea of checksums.

The reason we're talking about double write buffers in this thread is that double write buffers can be used to solve the problem with hint bits and checksums.

You're right, though, that it's academical whether double write buffers can be used without checksums on data pages, if the whole point of the exercise is to make it possible to have checksums on data pages..

  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com

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