Re: [HACKERS] 9.4.1 -> 9.4.2 problem: could not access status of transaction 1

On 2015-06-08 15:15:04 +0200, Andres Freund wrote:
> 1) the autovacuum trigger logic isn't perfect yet. I.e. especially with
>   autovacuum=off you can get into situations where emergency vacuums
>   aren't started when necessary. This is particularly likely to happen
>   if either very large multixacts are used, or if the server has been
>   shut down while emergency autovacuum where happening. No corruption
>   ensues, but it's not easy to get out of.

A first version to address this problem can be found appended to this
email.

Basically it does:
* Whenever more than MULTIXACT_MEMBER_SAFE_THRESHOLD are used, signal
  autovacuum once per members segment
* For both members and offsets, once hitting the hard limits, signal
  autovacuum everytime. Otherwise we loose the information when
  restarting the database, or when autovac is killed. I ran into this a
  bunch of times while testing.

Regards,

Andres