On Thu, Dec 29, 2011 at 4:44 PM, Kevin Grittner
<kevin.gritt...@wicourts.gov> wrote:
>> Heikki Linnakangas  wrote:
>> On 28.12.2011 01:39, Simon Riggs wrote:
>>> On Tue, Dec 27, 2011 at 8:05 PM, Heikki Linnakangas
>>>  wrote:
>>>> On 25.12.2011 15:01, Kevin Grittner wrote:
>>>>> I don't believe that. Double-writing is a technique to avoid
>>>>> torn pages, but it requires a checksum to work. This chicken-
>>>>> and-egg problem requires the checksum to be implemented first.
>>>> I don't think double-writes require checksums on the data pages
>>>> themselves, just on the copies in the double-write buffers. In
>>>> the double-write buffer, you'll need some extra information per-
>>>> page anyway, like a relfilenode and block number that indicates
>>>> which page it is in the buffer.
> You are clearly right -- if there is no checksum in the page itself,
> you can put one in the double-write metadata.  I've never seen that
> discussed before, but I'm embarrassed that it never occurred to me.

Heikki's idea for double writes works well. It solves the problems of
torn pages in a way that would make FPW redundant.

However, I don't see that it provides protection across non-crash
write problems. We know we have these since many systems have run
without a crash for years and yet still experience corrupt data.

Double writes do not require page checksums but neither do they
replace page checksums.

So I think we need page checksums plus either FPWs or double writes.

 Simon Riggs                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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