From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>, Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com>, Peter Geoghegan <pg(at)heroku(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, Petr Jelinek <petr(at)2ndquadrant(dot)com>, Greg S <stark(at)mit(dot)edu>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Freeze avoidance of very large table. |
Date: | 2016-03-08 17:49:24 |
Message-ID: | 825.1457459364@sss.pgh.pa.us |
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Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> The patch makes some attempt to update the comment mechanically, but
> that's not nearly enough. That comment is explaining that you *can't*
> rely on the visibility map to tell you *for sure* that a page does not
> require vacuuming. For current uses, that's OK, because if we miss a
> page we'll pick it up later. But now we want to skip vacuuming pages
> for relfrozenxid/relminmxid advancement, that rationale doesn't apply.
> Missing pages that need to be frozen and advancing relfrozenxid anyway
> would be _bad_.
Check.
> However, after some further thought, I think we might actually be OK.
> If a page goes from all-frozen to not-all-frozen while VACUUM is
> running, any new XID added to the page must be newer than the
> oldestXmin value computed by vacuum_set_xid_limits(), so it won't
> affect the value to which we can safely set relfrozenxid. Similarly,
> any MXID added to the page will be newer than GetOldestMultiXactId(),
> so setting relminmxid is still safe for similar reasons.
Yeah, I agree with this, as long as the issue is only that the visibility
map result is slightly stale and not that it's, say, not crash-safe.
We can reasonably assume that any newly-added XID must be one that was
in progress while VACUUM was running, and hence will be after the xmin
horizon we computed earlier. This requires the existence of a read
barrier somewhere between computing xmin horizon and inspecting the
visibility map, but I find it hard to believe there aren't plenty.
regards, tom lane
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