Re: tackling full page writes

On Thu, May 26, 2011 at 12:38 AM, Fujii Masao  wrote:
> On Thu, May 26, 2011 at 1:18 PM, Robert Haas  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