FAQ

On 7 June 2013 19:56, Heikki Linnakangas wrote:
On 07.06.2013 21:33, Simon Riggs wrote:

Now that I consider Greg's line of thought, the idea we focused on
here was about avoiding freezing. But Greg makes me think that we may
also wish to look at allowing queries to run longer than one epoch as
well, if the epoch wrap time is likely to come down substantially.

To do that I think we'll need to hold epoch for relfrozenxid as well,
amongst other things.
The biggest problem I see with that is that if a snapshot can be older than
2 billion XIDs, it must be possible to store XIDs on the same page that are
more than 2 billion XIDs apart. All the discussed schemes where we store the
epoch at the page level, either explicitly or derived from the LSN, rely on
the fact that it's not currently necessary to do that. Currently, when one
XID on a page is older than 2 billion XIDs, that old XID can always be
replaced with FrozenXid, because there cannot be a snapshot old enough to
not see it.
It does seem that there are two problems: avoiding freezing AND long
running queries

The long running query problem hasn't ever been looked at, it seems,
until here and now.
I agree that it would be nice if you could find a way around that. You had a
suggestion on making room on the tuple header for the epoch. I'm not sure I
like that particular proposal, but we would need something like that. If we
settle for snapshots that can be at most, say, 512 billion transactions old,
instead of 2 billion, then we would only need one byte to store an epoch
"offset" in the tuple header. Ie. deduce the epoch of tuples on the page
from the LSN on the page header, but allow individual tuples to specify an
offset from that deduced epoch.
I like the modification you propose. And I like it even better because
it uses just 1 byte, which is even more easily squeezed into the
existing tuple header, whether we go with my proposed squeezing route
or not.
In practice, I think we're still quite far from people running into that 2
billion XID limit on snapshot age. But maybe in a few years, after we've
solved all the more pressing vacuum and wraparound issues that people
currently run into before reaching that stage...
Your WALInsert lock patch will fix that. ;-)

--
  Simon Riggs http://www.2ndQuadrant.com/
  PostgreSQL Development, 24x7 Support, Training & Services

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