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.
- Heikki
--
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers
