On Wed, Dec 28, 2011 at 5:45 PM, Heikki Linnakangas
> On 28.12.2011 11:22, Simon Riggs wrote:
>> On Wed, Dec 28, 2011 at 7:42 AM, Heikki Linnakangas
>> <heikki(dot)linnakangas(at)enterprisedb(dot)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
>>> that has a valid checksum, you copy to the main storage. There's no need
>>> 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
Torn pages are not the only problem we need to detect.
You said "You scan the double write buffer...". When exactly would you do that?
Please explain how a double write buffer detects problems that do not
occur as the result of a crash.
We don't have much time, so please be clear and lucid.
Simon Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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