| From: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Petr Jelinek <petr(at)2ndquadrant(dot)com>, Steve Singer <steve(at)ssinger(dot)info>, Andres Freund <andres(at)2ndquadrant(dot)com>, Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Anssi Kääriäinen <anssi(dot)kaariainen(at)thl(dot)fi>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>, Jaime Casanova <jaime(at)2ndquadrant(dot)com> |
| Subject: | Re: tracking commit timestamps |
| Date: | 2014-11-10 13:40:04 |
| Message-ID: | 20141110134004.GA1791@alvin.alvh.no-ip.org |
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| Lists: | pgsql-hackers pgsql-www |
Robert Haas wrote:
> On Sun, Nov 9, 2014 at 8:41 PM, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote:
> > Robert Haas wrote:
> >> I think the key question here is the time for which the data needs to
> >> be retained. 2^32 of anything is a lot, but why keep around that
> >> number of records rather than more (after all, we have epochs to
> >> distinguish one use of a given txid from another) or fewer?
> >
> > The problem is not how much data we retain; is about how much data we
> > can address.
>
> I thought I was responding to a concern about disk space utilization.
Ah, right. So AFAIK we don't need to keep anything older than
RecentXmin or something like that -- which is not too old. If I recall
correctly Josh Berkus was saying in a thread about pg_multixact that it
used about 128kB or so in <= 9.2 for his customers; that one was also
limited to RecentXmin AFAIR. I think a similar volume of commit_ts data
would be pretty acceptable. Moreso considering that it's turned off by
default.
--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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