Simon Riggs wrote:
On Wed, 2008-10-22 at 12:29 +0300, Heikki Linnakangas wrote:
Simon Riggs wrote:
On Thu, 2008-10-16 at 18:52 +0300, Heikki Linnakangas wrote:
Simon Riggs wrote:
* The backend slot may not be reused for some time, so we should take
additional actions to keep state current and true. So we choose to log a
snapshot from the master into WAL after each checkpoint. This can then
be used to cleanup any unobserved xids. It also provides us with our
initial state data, see later.
We don't need to log a complete snapshot, do we? Just oldestxmin should
be enough.
Possibly, but you're thinking that once we're up and running we can use
less info.
Trouble is, you don't know when/if the standby will crash/be shutdown.
So we need regular full snapshots to allow it to re-establish full
information at regular points. So we may as well drop the whole snapshot
to WAL every checkpoint. To do otherwise would mean more code and less
flexibility.
Surely it's less code to write the OldestXmin to the checkpoint record,
rather than a full snapshot, no? And to read it off the checkpoint record.
You may be missing my point.
We need an initial state to work from.
I am proposing we write a full snapshot after each checkpoint because it
allows us to start recovery again from that point. If we wrote only
OldestXmin as you suggest it would optimise the size of the WAL record
but it would prevent us from restarting at that point.
Well, you'd just need to treat anything > oldestxmin, and not marked as
finished in clog, as unobserved. Which doesn't seem too bad. Not that
storing the full list of in-progress xids is that bad either, though.
Hmm. What about in-progress subtransactions that have overflowed the
shared mem cache? Can we rely that subtrans is up-to-date, up to the
checkpoint record that recovery starts from?
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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