On Thu, May 26, 2011 at 12:38 AM, Fujii Masao <masao.fu...@gmail.com> wrote: > On Thu, May 26, 2011 at 1:18 PM, Robert Haas <robertmh...@gmail.com> wrote: >>> The replay of the WAL record for A doesn't rely on the content of chunk 1 >>> which B modified. So I don't think that "partial page writes" has such >>> a problem. >>> No? >> >> Sorry. WAL records today DO rely on the prior state of the page. If >> they didn't, we wouldn't need full page writes. They don't rely on >> them terribly heavily - things like where pd_upper is pointing, and >> what the page LSN is. But they do rely on them. > > Yeah, I'm sure that normal WAL record (neither full page writes nor > "partial page writes") relies on the prior state of the page. But WAL > record for A is "partial page writes", which also relies on the prior > state?
Yeah, that's how it shakes out. The idea is you have to write the parts of the page that you rely on, but not the rest - which in turn guarantees that those parts (but not the rest) will be correct when you read them. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
