On Wed, Dec 28, 2011 at 5:45 PM, Heikki Linnakangas <heikki.linnakan...@enterprisedb.com> wrote: > 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.
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 -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers