24.1.5.1. Multixacts And Wraparound
The following documentation comment has been logged on the website:
Page: https://www.postgresql.org/docs/13/routine-vacuuming.html
Description:
In section "24.1.5.1. Multixacts And Wraparound" of
https://www.postgresql.org/docs/current/routine-vacuuming.html we find the
following sentence towards the end of the section:
"...If the amount of used member storage space exceeds the amount 50% of the
addressable storage space."
I am having trouble parsing "...exceeds the amount 50% of the
addressable..." part.
It seems like the sentence is trying to say "...space exceeds 50% of the
addressable storage space" or maybe even "...space exceeds the amount of the
addressable storage space". Which is which?
Many thanks,
Eric Mutta.
On Thu, 2021-06-17 at 18:31 +0000, PG Doc comments form wrote:
In section "24.1.5.1. Multixacts And Wraparound" of
https://www.postgresql.org/docs/current/routine-vacuuming.html we find the
following sentence towards the end of the section:"...If the amount of used member storage space exceeds the amount 50% of the
addressable storage space."I am having trouble parsing "...exceeds the amount 50% of the
addressable..." part.It seems like the sentence is trying to say "...space exceeds 50% of the
addressable storage space" or maybe even "...space exceeds the amount of the
addressable storage space". Which is which?
+1
I think that the second "the amount" in the sentence should be removed.
Yours,
Laurenz Albe
On Fri, Jun 18, 2021 at 08:51:00AM +0200, Laurenz Albe wrote:
On Thu, 2021-06-17 at 18:31 +0000, PG Doc comments form wrote:
In section "24.1.5.1. Multixacts And Wraparound" of
https://www.postgresql.org/docs/current/routine-vacuuming.html we find the
following sentence towards the end of the section:"...If the amount of used member storage space exceeds the amount 50% of the
addressable storage space."I am having trouble parsing "...exceeds the amount 50% of the
addressable..." part.It seems like the sentence is trying to say "...space exceeds 50% of the
addressable storage space" or maybe even "...space exceeds the amount of the
addressable storage space". Which is which?+1
I think that the second "the amount" in the sentence should be removed.
I think it is worse than that. Here is the full paragraph:
As a safety device, an aggressive vacuum scan will occur for any table
whose multixact-age is greater than
<xref linkend="guc-autovacuum-multixact-freeze-max-age"/>. Aggressive
vacuum scans will also occur progressively for all tables, starting with
those that have the oldest multixact-age, if the amount of used member
storage space exceeds the amount 50% of the addressable storage space.
Both of these kinds of aggressive scans will occur even if autovacuum is
nominally disabled.
What does "the amount of used member storage space exceeds 50% of the
addressable storage space" mean?
--
Bruce Momjian <bruce@momjian.us> https://momjian.us
EDB https://enterprisedb.com
If only the physical world exists, free will is an illusion.
On Mon, 2021-06-21 at 22:50 -0400, Bruce Momjian wrote:
On Fri, Jun 18, 2021 at 08:51:00AM +0200, Laurenz Albe wrote:
On Thu, 2021-06-17 at 18:31 +0000, PG Doc comments form wrote:
In section "24.1.5.1. Multixacts And Wraparound" of
https://www.postgresql.org/docs/current/routine-vacuuming.html we find the
following sentence towards the end of the section:"...If the amount of used member storage space exceeds the amount 50% of the
addressable storage space."I am having trouble parsing "...exceeds the amount 50% of the
addressable..." part.I think that the second "the amount" in the sentence should be removed.
I think it is worse than that. Here is the full paragraph:
As a safety device, an aggressive vacuum scan will occur for any table
whose multixact-age is greater than
<xref linkend="guc-autovacuum-multixact-freeze-max-age"/>. Aggressive
vacuum scans will also occur progressively for all tables, starting with
those that have the oldest multixact-age, if the amount of used member
storage space exceeds the amount 50% of the addressable storage space.
Both of these kinds of aggressive scans will occur even if autovacuum is
nominally disabled.What does "the amount of used member storage space exceeds 50% of the
addressable storage space" mean?
You are right. See MultiXactMemberFreezeThreshold for the whole story.
What about:
As a safety device, an aggressive vacuum scan will occur for any table
whose multixact-age (see <xref linkend="vacuum-for-multixact-wraparound"/>)
is greater than <xref linkend="guc-autovacuum-multixact-freeze-max-age"/>.
Also, if the storage occupied by multixacts exceeds 2GB, aggressive vacuum
scans will occur more often for all tables, starting with those that have
the oldest multixact-age.
Both of these kinds ...
I hope I read the source right concerning the 2GB.
Yours,
Laurenz Albe
On Tue, Jun 22, 2021 at 08:32:18AM +0200, Laurenz Albe wrote:
What does "the amount of used member storage space exceeds 50% of the
addressable storage space" mean?You are right. See MultiXactMemberFreezeThreshold for the whole story.
What about:
As a safety device, an aggressive vacuum scan will occur for any table
whose multixact-age (see <xref linkend="vacuum-for-multixact-wraparound"/>)
is greater than <xref linkend="guc-autovacuum-multixact-freeze-max-age"/>.
Also, if the storage occupied by multixacts exceeds 2GB, aggressive vacuum
scans will occur more often for all tables, starting with those that have
the oldest multixact-age.
Both of these kinds ...
Yes, very good. There were three problems with the original paragraph:
* Had duplicate words
* Had awkward phrasing
* Used undefined terms
Your version fixes all three of those. Patch attached.
I hope I read the source right concerning the 2GB.
I can confirm the 2GB. do_vacuum() calls
MultiXactMemberFreezeThreshold() to set its
effective_multixact_freeze_max_age. MultiXactMemberFreezeThreshold()
compares the number of members to 2^32 (0xFFFFFFFF) / 2 or 2GB:
#define MaxMultiXactOffset ((MultiXactOffset) 0xFFFFFFFF)
#define MULTIXACT_MEMBER_SAFE_THRESHOLD (MaxMultiXactOffset / 2)
/* If member space utilization is low, no special action is required. */
if (members <= MULTIXACT_MEMBER_SAFE_THRESHOLD)
return autovacuum_multixact_freeze_max_age;
If that test fails, autovacuum_multixact_freeze_max_age is set much more
aggressively.
--
Bruce Momjian <bruce@momjian.us> https://momjian.us
EDB https://enterprisedb.com
If only the physical world exists, free will is an illusion.
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On 2021-Jun-24, Bruce Momjian wrote:
+ As a safety device, an aggressive vacuum scan will + occur for any table whose multixact-age (see <xref + linkend="vacuum-for-multixact-wraparound"/>) is greater than <xref + linkend="guc-autovacuum-multixact-freeze-max-age"/>. Also, if the + storage occupied by multixacts exceeds 2GB, aggressive vacuum scans + will occur more often for all tables, starting with those that have + the oldest multixact-age. Both of these kinds of aggressive scans + will occur even if autovacuum is nominally disabled.
This looks good, thanks.
I think "the space occupied by multixacts" is a bit ambiguous -- it is
talking about pg_multixact/members only, but you could interpret that it
talks about both that and pg_multixact/offsets. I'm not sure we need to
be 100% precise about that, so perhaps what you have is sufficient. But
if we do want to be precise, then maybe " ... if the storage occupied by
multixact members (<literal>pg_multixact/members/</literal>) exceeds ..."
covers it.
(At least, that's how I remember this. I don't think things have
changed much since 53bb309d2d5a ...)
--
�lvaro Herrera Valdivia, Chile
On Thu, Jun 24, 2021 at 04:06:38PM -0400, Álvaro Herrera wrote:
On 2021-Jun-24, Bruce Momjian wrote:
+ As a safety device, an aggressive vacuum scan will + occur for any table whose multixact-age (see <xref + linkend="vacuum-for-multixact-wraparound"/>) is greater than <xref + linkend="guc-autovacuum-multixact-freeze-max-age"/>. Also, if the + storage occupied by multixacts exceeds 2GB, aggressive vacuum scans + will occur more often for all tables, starting with those that have + the oldest multixact-age. Both of these kinds of aggressive scans + will occur even if autovacuum is nominally disabled.This looks good, thanks.
I think "the space occupied by multixacts" is a bit ambiguous -- it is
talking about pg_multixact/members only, but you could interpret that it
talks about both that and pg_multixact/offsets. I'm not sure we need to
be 100% precise about that, so perhaps what you have is sufficient. But
if we do want to be precise, then maybe " ... if the storage occupied by
multixact members (<literal>pg_multixact/members/</literal>) exceeds ..."
covers it.(At least, that's how I remember this. I don't think things have
changed much since 53bb309d2d5a ...)
OK, I got "members" into the doc section and applied it to master and PG
14.
--
Bruce Momjian <bruce@momjian.us> https://momjian.us
EDB https://enterprisedb.com
If only the physical world exists, free will is an illusion.