Re: commit dfda6ebaec67 versus wal_keep_segments

On 03.04.2013 22:50, Jeff Janes wrote:
> On Wed, Apr 3, 2013 at 11:14 AM, Heikki Linnakangas<hlinnakangas@vmware.com
>> wrote:
>
>> On 03.04.2013 18:58, Jeff Janes wrote:
>>
>>> On Tue, Apr 2, 2013 at 10:08 PM, Jeff Janes<jeff.janes@gmail.com>   wrote:
>>>
>>>   This commit introduced a problem with wal_keep_segments:
>>>>
>>>> commit dfda6ebaec6763090fb78b458a979b**558c50b39b
>>>>
>>>
>>> The problem seems to be that the underflow warned about is happening,
>>> because the check to guard it was checking the wrong thing.  However, I
>>> don't really understand KeepLogSeg.  It seems like segno, and hence
>>> recptr,
>>> don't actually serve any purpose.
>>>
>>
>> Hmm, the check is actually correct, but the assignment in the else-branch
>> isn't. The idea of KeepLogSeg is to calculate recptr - wal_keep_segments,
>> and assign that to *logSegNo. But only if *logSegNo is not already<  than
>> the calculated value. Does the attached look correct to you?
>
>
> Let me describe what I think is going on.  My description is "On start,
> recptr is the redo location of the just-completed checkpoint, and logSegNo
> is the redo location segment of the checkpoint before that one.  We want to
> keep the previous-checkpoint redo location, and we also want to keep
> wal_keep_segments before the current-checkpoint redo location, so we take
> whichever is earlier."
>
> If my understanding is now correct, then I think your patch looks correct.
>   (Also, applying it fixed the problem I was having.)

Ok, thanks, applied.

> Why do we keep wal_keep_segments before the just-finished checkpoint,
> rather than keeping that many before the previous checkpoint?  I seems like
> it would be more intuitive (to the DBA) for that parameter to mean "keep
> this many more segments than you otherwise would".  I'm not proposing we
> change it, I'm just curious about why it is done that way.

It feels more intuitive to me the way it is. wal_keep_log_segments means 
"make sure there are always this many old WAL segments available in the 
server, regardless of any other settings". If you have a standby, it 
means that you don't need a new base backup as long as you don't fall 
behind the master by more than wal_keep_segments segments.

On 03.04.2013 21:33, Alvaro Herrera wrote:
> "by by"

Fixed, thanks.

- Heikki