| From: | Heikki Linnakangas <hlinnakangas(at)vmware(dot)com> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | PostgreSQL-development <pgsql-hackers(at)postgreSQL(dot)org>, Andres Freund <andres(at)2ndquadrant(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com> |
| Subject: | Re: Spreading full-page writes |
| Date: | 2014-05-26 17:22:38 |
| Message-ID: | 32349dda-e62c-4a13-940c-1268bbca823e@email.android.com |
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| Lists: | pgsql-hackers |
On 26 May 2014 20:16:33 EEST, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>On May 25, 2014, at 5:52 PM, Heikki Linnakangas
><hlinnakangas(at)vmware(dot)com> wrote:
>> Here's how this works out during replay:
>>
>> a) You start WAL replay from the latest checkpoint's Redo-pointer.
>>
>> When you see a WAL record that's been marked with XLR_FPW_SKIPPED,
>don't replay that record at all. It's OK because we know that there
>will be a separate record containing the full-page image of the page
>later in the stream.
>
>I don't think we know that. The server might have crashed before that
>second record got generated. (This appears to be an unfixable flaw in
>this proposal.)
The second record is generated before the checkpoint is finished and the checkpoint record is written. So it will be there.
(if you crash before the checkpoint is finished, the in-progress checkpoint is no good for recovery anyway, and won't be used)
- Heikki
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