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#1 (**permalink**) 04-12-2008, 07:15 AM

Hi

I'm trying to gain a better understanding of how the postgres
xlog works - especially about the corner cases of wal replay.

One thing that I do not understand is what CheckPoint.undo is
used for. I grepped through the source, and only see very few
references to it, which either just print it, or set it to
the checkpoint location or the redo location.

The only place where is is checked is
if (XLByteLT(checkPoint.undo, RecPtr) || XLByteLT(checkPoint.redo, RecPtr))
but I don't see the reason for this check, given that undo never
seems to be assigned any interesting value..

Is there any in-depth documentation about the postgres xlog
implementation?
What am I missing?

greetings, Florian Pflug

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#2 (**permalink**) 04-12-2008, 07:15 AM

> What am I missing?
Seems, it's about that
http://archives.postgresql.org/pgsql...6/msg00085.php

--
Teodor Sigaev E-mail: teodor@sigaev.ru
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#3 (**permalink**) 04-12-2008, 07:15 AM

Opps, sorry, I missed checkpoint keyword

Teodor Sigaev wrote:
>
>> What am I missing?
> Seems, it's about that
> http://archives.postgresql.org/pgsql...6/msg00085.php
>

--
Teodor Sigaev E-mail: teodor@sigaev.ru
WWW: http://www.sigaev.ru/

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#4 (**permalink**) 04-12-2008, 07:15 AM

No you're right, it's related to the WAL undo stuff that was never
actually implemented. It's dead code.

Teodor Sigaev wrote:
> Opps, sorry, I missed checkpoint keyword
>
> Teodor Sigaev wrote:
>>
>>> What am I missing?
>> Seems, it's about that
>> http://archives.postgresql.org/pgsql...6/msg00085.php
>>
>

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com

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#5 (**permalink**) 04-12-2008, 07:15 AM

Heikki Linnakangas wrote:
> No you're right, it's related to the WAL undo stuff that was never
> actually implemented. It's dead code.
>
> Teodor Sigaev wrote:
>> Opps, sorry, I missed checkpoint keyword
>>
>> Teodor Sigaev wrote:
>>>
>>>> What am I missing?
>>> Seems, it's about that
>>> http://archives.postgresql.org/pgsql...6/msg00085.php

Thanks - everything makes much more sense now ;-)

greetings, Florian Pflug

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#6 (**permalink**) 04-12-2008, 07:15 AM

Florian G. Pflug wrote:
> Heikki Linnakangas wrote:
> >No you're right, it's related to the WAL undo stuff that was never
> >actually implemented. It's dead code.
> >
> >Teodor Sigaev wrote:
> >>Opps, sorry, I missed checkpoint keyword
> >>
> >>Teodor Sigaev wrote:
> >>>
> >>>>What am I missing?
> >>>Seems, it's about that
> >>>http://archives.postgresql.org/pgsql...6/msg00085.php
>
> Thanks - everything makes much more sense now ;-)

So if you want to submit a patch to remove it, it'll make sense for
future readers as well ;-)

--
Alvaro Herrera http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.

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#7 (**permalink**) 04-12-2008, 07:16 AM

Florian G. Pflug wrote:
> Alvaro Herrera wrote:
> >Florian G. Pflug wrote:
> >>Heikki Linnakangas wrote:
> >>>No you're right, it's related to the WAL undo stuff that was never
> >>>actually implemented. It's dead code.
> >>>
> >>>Teodor Sigaev wrote:
> >>>>Opps, sorry, I missed checkpoint keyword
> >>>>
> >>>>Teodor Sigaev wrote:
> >>>>>>What am I missing?
> >>>>>Seems, it's about that
> >>>>>http://archives.postgresql.org/pgsql...6/msg00085.php
> >>Thanks - everything makes much more sense now ;-)
> >
> >So if you want to submit a patch to remove it, it'll make sense for
> >future readers as well ;-)
>
> Here you go. I'm not subscribe to pgsql-patches, so I posted this here.

I think you should increase pg_control version.

--
Alvaro Herrera http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.

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#8 (**permalink**) 04-12-2008, 07:16 AM

Alvaro Herrera <alvherre@commandprompt.com> writes:
> I think you should increase pg_control version.

And the WAL page-header version, since this also changes WAL contents.

regards, tom lane

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What is CheckPoint.undo needed for?