Re: How should pg_standby get over the gap of timeline?

Hi, Simon. Thanks for the comment!!

On Sat, Nov 22, 2008 at 2:09 AM, Simon Riggs <simon@2ndquadrant.com> wrote:
>
> On Thu, 2008-11-20 at 22:41 +0900, Fujii Masao wrote:
>
>> In the current Synch Rep patch, the standby cannot catch up with the
>> primary which has a bigger timeline. So, whenever making the standby
>> catch up, a fresh base backup is required. This is obviously undesirable,
>> and I'd like to get rid of this restriction.
>>
>> Postgres itself can recover up to a bigger timeline without a base
>> backup. The remaining problem is that pg_standby cannot get over the
>> gap of timeline. It continues waiting for the XLOG file with out-of-date
>> timeline, and redo doesn't progress.
>
> We've discussed this before. My answer is the same: you are assuming it
> is safe to re-enter recovery, which is not correct (currently).

I'm afraid you might be right. But I cannot understand yet why it's not
safe to re-enter recovery. Is it safe to re-enter recovery from the
restart point after PITR stopped halfway? If it's safe, ISTM that PITR
without a base backup also is safe. Please let me know what might
violate a re-entry of recovery. What is your worry?

> You are
> also assuming that taking a base backup is an expensive operation - it
> need not be so if you simply move only the files/data that have changed,
> e.g. rsync.

It depends on DB size and type. I think that it's important that the user
*can* choose the better method according to his situation.

Regards,

-- 
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center