Date: Wed, 2 Jul 2014 13:11:51 -0400
From: Bruce Momjian <bruce@momjian.us>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Andres Freund <andres@2ndquadrant.com>,
 PostgreSQL Bugs <pgsql-bugs@postgresql.org>
Subject: Re: pg_upgrade < 9.3 -> >=9.3 misses a step around
 multixacts
Message-ID: <20140702171151.GA20463@momjian.us>
References: <20140619020633.GA20469@momjian.us>
 <20140619015205.GD3806@momjian.us>
 <20140619014034.GC3806@momjian.us>
 <20140619220425.GQ18688@eldon.alvh.no-ip.org>
 <20140620010653.GS18688@eldon.alvh.no-ip.org>
 <20140620180725.GD29143@momjian.us>
 <20140624201346.GB17110@momjian.us>
 <20140627210520.GP7340@eldon.alvh.no-ip.org>
 <20140701014917.GA21190@momjian.us>
 <20140702155634.GD7340@eldon.alvh.no-ip.org>
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On Wed, Jul  2, 2014 at 11:56:34AM -0400, Alvaro Herrera wrote:
> > How are we not creating a gap in members files by setting the members
> > value?  Why will that not cause problems?
> 
> We're not setting the offset and member value here; we're setting the
> "nextMulti" and "oldestMulti" values.  Both affect the offsets counter,
> not the members counter.  The members counter is kept at zero, so the
> next multi to be allocated will create members starting from the first
> members position in page zero.  initdb of the new cluster created the
> first members page, which corresponds to the first members value that
> will be used.
> 
> Note pg_resetxlog --help says:
> 
>   -m MXID,MXID     set next and oldest multitransaction ID
> 
> I think you're confusing the fact that we pass two values here (next and
> oldest, both apply to offset counters) with offsets vs. members.
> 
> To affect the members counter you would use this other pg_resetxlog switch:
>   -O OFFSET        set next multitransaction offset
> which, AFAICS, we only use when upgrading from a 9.3+ cluster to a newer
> 9.3+ cluster (and we also copy the files from old cluster to the new
> one).

OK, thanks for the analysis.  Attached patch applied back through 9.3.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + Everyone has their own god. +

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diff --git a/contrib/pg_upgrade/pg_upgrade.c b/contrib/pg_upgrade/pg_upgrade.c
new file mode 100644
index b32d81e..5e715fc
*** a/contrib/pg_upgrade/pg_upgrade.c
--- b/contrib/pg_upgrade/pg_upgrade.c
*************** copy_clog_xlog_xid(void)
*** 451,461 ****
  	else if (new_cluster.controldata.cat_ver >= MULTIXACT_FORMATCHANGE_CAT_VER)
  	{
  		/*
! 		 * Remove files created by initdb that no longer match the
! 		 * new multi-xid value.
  		 */
  		remove_new_subdir("pg_multixact/offsets", false);
- 		remove_new_subdir("pg_multixact/members", false);
  
  		prep_status("Setting oldest multixact ID on new cluster");
  
--- 451,461 ----
  	else if (new_cluster.controldata.cat_ver >= MULTIXACT_FORMATCHANGE_CAT_VER)
  	{
  		/*
! 		 * Remove offsets/0000 file created by initdb that no longer matches
! 		 * the new multi-xid value.  "members" starts at zero so no need to
! 		 * remove it.
  		 */
  		remove_new_subdir("pg_multixact/offsets", false);
  
  		prep_status("Setting oldest multixact ID on new cluster");
  

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