Re: tracking commit timestamps

On 11/10/14, 7:40 AM, Alvaro Herrera wrote:
> 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.

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