On Thu, Dec 18, 2014 at 10:49 AM, Heikki Linnakangas <hlinnakan...@vmware.com> wrote: > On 12/18/2014 03:53 PM, Robert Haas wrote: >> On Wed, Dec 17, 2014 at 8:40 AM, Heikki Linnakangas >> <hlinnakan...@vmware.com> wrote: >>> >>> At the end of archive recovery, we copy the last segment from the old >>> timeline, to initialize the first segment on the new timeline. For >>> example, >>> if the timeline switch happens in the middle of WAL segment >>> 000000010000000000000005, the whole 000000010000000000000005 segment is >>> copied to become 000000020000000000000005. The copying is necessary, so >>> that >>> the new segment contains valid data up to the switch point. >>> >>> However, we wouldn't really need to copy the whole segment, copying up to >>> the switch point would be enough. In fact, copying the whole segment is a >>> bad idea, because the copied WAL looks valid on the new timeline too. >> >> >> Your proposed change makes sense to me, but why do we need the segment >> to contain valid data up to the switch point? It seems like the >> switch between timelines should be "crisper": replay WAL on the old >> timeline only from the old segment, and from the new timeline only on >> the new segment. Anything else seems like an invitation to unending >> corner-case bugs. > > True. That would require some changes to the way archive recovery works, > though. Currently, when our recovery target timeline is, for example, 5, > whose parents are 4 and 3, and we're currently on timeline 3, we will try to > restore each segment first with timeline ID 5, then 4, then 3. It's a bit > silly, because we know the timeline history and the exact points where the > timelines changed, so we could just fetch the correct one. That would be a > good idea, but I'm going to go ahead with just this smaller change now.
Yeah, it's a separate issue. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
