Prevent archive recovery from scanning non-existent WAL files.

MasaoFujii · MasaoFujii · commit 660f1fc40d7a · 2020-05-09T12:11:11.000+09:00
Previously when there were multiple timelines listed in the history file of the recovery target timeline, archive recovery searched all of them, starting from the newest timeline to the oldest one, to find the segment to read. That is, archive recovery had to continuously fail scanning the segment until it reached the timeline that the segment belonged to. These scans for non-existent segment could be harmful on the recovery performance especially when archival area was located on the remote storage and each scan could take a long time. To address the issue, this commit changes archive recovery so that it skips scanning the timeline that the segment to read doesn't belong to. Per discussion, back-patch to all supported versions. Author: Kyotaro Horiguchi, tweaked a bit by Fujii Masao Reviewed-by: David Steele, Pavel Suderevsky, Grigory Smolkin Discussion: https://postgr.es/m/16159-f5a34a3a04dc67e0@postgresql.org Discussion: https://postgr.es/m/20200129.120222.1476610231001551715.horikyota.ntt@gmail.com
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
@@ -3692,11 +3692,36 @@ XLogFileReadAnyTLI(XLogSegNo segno, int emode, int source)
 
 	foreach(cell, tles)
 	{
-		TimeLineID	tli = ((TimeLineHistoryEntry *) lfirst(cell))->tli;
+		TimeLineHistoryEntry *hent = (TimeLineHistoryEntry *) lfirst(cell);
+		TimeLineID	tli = hent->tli;
 
 		if (tli < curFileTLI)
 			break;				/* don't bother looking at too-old TLIs */
 
+		/*
+		 * Skip scanning the timeline ID that the logfile segment to read
+		 * doesn't belong to
+		 */
+		if (hent->begin != InvalidXLogRecPtr)
+		{
+			XLogSegNo	beginseg = 0;
+
+			XLByteToSeg(hent->begin, beginseg);
+
+			/*
+			 * The logfile segment that doesn't belong to the timeline is
+			 * older or newer than the segment that the timeline started or
+			 * ended at, respectively. It's sufficient to check only the
+			 * starting segment of the timeline here. Since the timelines are
+			 * scanned in descending order in this loop, any segments newer
+			 * than the ending segment should belong to newer timeline and
+			 * have already been read before. So it's not necessary to check
+			 * the ending segment of the timeline here.
+			 */
+			if (segno < beginseg)
+				continue;
+		}
+
 		if (source == XLOG_FROM_ANY || source == XLOG_FROM_ARCHIVE)
 		{
 			fd = XLogFileRead(segno, emode, tli,
