Message-ID: <4AC49281.2040302@enterprisedb.com>
Date: Thu, 01 Oct 2009 14:29:05 +0300
From: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>
Organization: EnterpriseDB
User-Agent: Mozilla-Thunderbird 2.0.0.19 (X11/20090103)
MIME-Version: 1.0
To: Simon Riggs <simon@2ndQuadrant.com>
CC: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Hot Standby on git
References: <1253958559.4449.833.camel@ebony.2ndQuadrant>
In-Reply-To: <1253958559.4449.833.camel@ebony.2ndQuadrant>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit

+   /*
+    * If our initial RunningXactData had an overflowed snapshot then we
+    * knew we were missing some subxids from our snapshot. We can use
+    * this data as an initial snapshot, but we cannot yet mark it valid.
+    * We know that the missing subxids are equal to or earlier than
+    * LatestRunningXid. After we initialise we continue to apply changes
+    * during recovery, so once the oldestRunningXid is later than the
+    * initLatestRunningXid we can now prove that we no longer have
+    * missing information and can mark the snapshot as valid.
+    */
+   if (initRunningXactData && !recoverySnapshotValid)
+   {
+       if (TransactionIdPrecedes(initLatestRunningXid,
xlrec->oldestRunningXid)
+       {
+           recoverySnapshotValid = true;
+           elog(trace_recovery(DEBUG2),
+                   "running xact data now proven complete");
+           elog(trace_recovery(DEBUG2),
+                   "recovery snapshots are now enabled");
+       }
+       return;
+   }
+

When GetRunningXactData() calculates latestRunningXid in the master,
which is stored in initLatestRunningXid in the standby, it only looks at
xids and subxids present in the procarray. It doesn't take into account
overflowed subxids. I think we could declare a recovery snapshot "proven
complete" too early because of that.

-- 
  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com