Re: Allow WAL information to recover corrupted pg_controldata

On Thu, Jun 14, 2012 at 11:39 AM, Amit Kapila <amit(dot)kapila(at)huawei(dot)com> wrote:
> I am planning to work on the below Todo list item for this CommitFest
> Allow WAL information to recover corrupted pg_controldata
> http://archives.postgresql.org/pgsql-patches/2006-06/msg00025.php

The deadline for patches for this CommitFest is today, so I think you
should target any work you're starting now for the NEXT CommitFest.

> I wanted to confirm my understanding about the work involved for this patch:
> The existing patch has following set of problems:
>    1. Memory leak and linked list code path is not proper
>    2. lock check for if the server is already running, is removed in patch
> which needs to be reverted
>    3. Refactoring of the code.
>
> Apart from above what I understood from the patch is that its intention is
> to generate values for ControlFile using WAL logs when -r option is used.
>
> The change in algorithm from current will be if control file is corrupt
> which essentialy means ReadControlFile() will return False, then it should
> generate values (checkPointCopy, checkPoint, prevCheckPoint, state) using
> WAL if -r option is enabled.
>
> Also for -r option, it doesn't need to call function FindEndOfXLOG() as the
> that work will be achieved by above point.
>
> It will just rewrite the control file and don’t do other resets.
>
>
> The algorithm of restoring the pg_control value from old xlog file:
>    1. Retrieve all of the active xlog files from xlog direcotry into a list
> by increasing order, according their timeline, log id, segment id.
>    2. Search the list to find the oldest xlog file of the lastest time line.
>    3. Search the records from the oldest xlog file of latest time line to
> the latest xlog file of latest time line, if the checkpoint record
>       has been found, update the latest checkpoint and previous checkpoint.
>
>
>
> Apart from above some changes in code will be required after the Xlog patch
> by Heikki.
>
> Suggest me if my understanding is correct?

I guess my first question is: why do we need this? There are lots of
things in the TODO list that someone wanted once upon a time, but
they're not all actually important. Do you have reason to believe
that this one is? It's been six years since that email, so it's worth
asking if this is actually relevant.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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