| From: | Heikki Linnakangas <hlinnakangas(at)vmware(dot)com> |
|---|---|
| To: | Andres Freund <andres(at)2ndquadrant(dot)com> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Jeff Davis <pgsql(at)j-davis(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: removing PD_ALL_VISIBLE |
| Date: | 2013-05-30 12:34:04 |
| Message-ID: | 51A7473C.6070208@vmware.com |
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| Lists: | pgsql-hackers |
On 30.05.2013 15:12, Andres Freund wrote:
> Now, I am far from being convinced its a good idea to get rid of
> PD_ALL_VISIBLE, but I don't think it does. Except that it currently is
> legal for the page level ALL_VISIBLE being set while the corresponding
> visibilitymap one isn't there's not much prohibiting us fundamentally
> from looking in the vm when we need to know whether the page is all
> visible, is there?
Hmm, so you're suggesting that the visibility map would be *required* to
interpret the pages correctly. Ie. if you just zapped the visibility
map, you'd lose critical information and the heap would appear to be
corrupt. I guess that's possible, but it makes me quite uneasy. At the
moment, it's relieving to know that it's always safe to just truncate
the visibility map in case of emergency.
- Heikki
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