On 11/10/14, 7:40 AM, Alvaro Herrera wrote:
Robert Haas wrote:
On Sun, Nov 9, 2014 at 8:41 PM, Alvaro Herrera <alvhe...@2ndquadrant.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.
FWIW, AFAICS MultiXacts are only truncated after a (auto)vacuum process is able
to advance datminmxid, which will (now) only happen when an entire relation has
been scanned (which should be infrequent).
I believe the low normal space usage is just an indication that most databases
don't use many MultiXacts.
--
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com
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