postgres.git / summary / log / commit / refs

commit    acf1dd42342d6d84ca8e7a1998335e2e8809759e
Author:   Thomas Munro <tmunro@postgresql.org>
Date:     Sat Apr 16 22:22:03 2022 +0000

    Don't retry restore_command while reading ahead.
    
    Suppress further attempts to read ahead in the WAL if we run out of
    data, until the records already decoded have been replayed.  This
    restores the traditional behavior for continuous archive recovery, which
    is to retry the failing restore_command only every 5 seconds.  With the
    coding in 5dc0418f, we would start retrying every time through the
    recovery loop when our WAL decoding window hit the end of the current
    segment and we tried to look ahead into a not-yet-available next file.
    That was very slow.
    
    Also change the no_readahead_until mechanism to use <= rather than <,
    which seems more useful.  Otherwise we'd either get one extra unwanted
    retry of restore_command, or we'd need to add 1 to an LSN.
    
    No change in behavior for regular streaming.  That was already limited
    by the flushedUpto variable, which won't be updated until we replay what
    we have already.
    
    Reported by Andres Freund while analyzing the failure of a TAP test on
    build farm animal skink (investigation ongoing but probably due to
    otherwise unrelated timing bugs triggered by this slowness magnified by
    valgrind).
    
    Discussion: https://postgr.es/m/20220409005910.alw46xqmmgny2sgr%40alap3.anarazel.de


 src/backend/access/transam/xlogprefetcher.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/src/backend/access/transam/xlogprefetcher.c b/src/backend/access/transam/xlogprefetcher.c
index f3428888d2b..959e4094667 100644
--- a/src/backend/access/transam/xlogprefetcher.c
+++ b/src/backend/access/transam/xlogprefetcher.c
@@ -487,8 +487,8 @@ XLogPrefetcherNextBlock(uintptr_t pgsr_private, XLogRecPtr *lsn)
 			 */
 			nonblocking = XLogReaderHasQueuedRecordOrError(reader);
 
-			/* Certain records act as barriers for all readahead. */
-			if (nonblocking && replaying_lsn < prefetcher->no_readahead_until)
+			/* Readahead is disabled until we replay past a certain point. */
+			if (nonblocking && replaying_lsn <= prefetcher->no_readahead_until)
 				return LRQ_NEXT_AGAIN;
 
 			record = XLogReadAhead(prefetcher->reader, nonblocking);
@@ -496,8 +496,13 @@ XLogPrefetcherNextBlock(uintptr_t pgsr_private, XLogRecPtr *lsn)
 			{
 				/*
 				 * We can't read any more, due to an error or lack of data in
-				 * nonblocking mode.
+				 * nonblocking mode.  Don't try to read ahead again until
+				 * we've replayed everything already decoded.
 				 */
+				if (nonblocking && prefetcher->reader->decode_queue_tail)
+					prefetcher->no_readahead_until =
+						prefetcher->reader->decode_queue_tail->lsn;
+
 				return LRQ_NEXT_AGAIN;
 			}
 


[parent: 4a736a161c30]