| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Simon Riggs <simon(at)2ndquadrant(dot)com>, Merlin Moncure <mmoncure(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: collateral benefits of a crash-safe visibility map |
| Date: | 2011-05-10 17:08:30 |
| Message-ID: | 18591.1305047310@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Tue, May 10, 2011 at 12:57 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>> Hmmm, do we really need to WAL log freezing?
> That might solve the relfrozenxid problem - set the bits in the heap,
> sync the heap, then update relfrozenxid once the heap is guaranteed
> safely on disk - but it again seems problematic for Hot Standby.
... or even warm standby. You basically *have to* WAL-log freezing
before you can truncate pg_clog. The only freedom you have here is
freedom to mess with the policy about how soon you try to truncate
pg_clog.
(Doing an unlogged freeze operation first is right out, too, if it
causes the system to fail to perform/log the operation later.)
regards, tom lane
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