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Commitacf1dd4

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Don't retry restore_command while reading ahead.
Suppress further attempts to read ahead in the WAL if we run out ofdata, until the records already decoded have been replayed. Thisrestores the traditional behavior for continuous archive recovery, whichis to retry the failing restore_command only every 5 seconds. With thecoding in5dc0418, we would start retrying every time through therecovery loop when our WAL decoding window hit the end of the currentsegment 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 unwantedretry of restore_command, or we'd need to add 1 to an LSN.No change in behavior for regular streaming. That was already limitedby the flushedUpto variable, which won't be updated until we replay whatwe have already.Reported by Andres Freund while analyzing the failure of a TAP test onbuild farm animal skink (investigation ongoing but probably due tootherwise unrelated timing bugs triggered by this slowness magnified byvalgrind).Discussion:https://postgr.es/m/20220409005910.alw46xqmmgny2sgr%40alap3.anarazel.de
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‎src/backend/access/transam/xlogprefetcher.c

Lines changed: 8 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -487,17 +487,22 @@ XLogPrefetcherNextBlock(uintptr_t pgsr_private, XLogRecPtr *lsn)
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*/
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nonblocking=XLogReaderHasQueuedRecordOrError(reader);
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/*Certain records act as barriers for all readahead. */
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if (nonblocking&&replaying_lsn<prefetcher->no_readahead_until)
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/*Readahead is disabled until we replay past a certain point. */
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if (nonblocking&&replaying_lsn <=prefetcher->no_readahead_until)
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returnLRQ_NEXT_AGAIN;
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record=XLogReadAhead(prefetcher->reader,nonblocking);
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if (record==NULL)
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{
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/*
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* We can't read any more, due to an error or lack of data in
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* nonblocking mode.
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* nonblocking mode. Don't try to read ahead again until
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* we've replayed everything already decoded.
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*/
502+
if (nonblocking&&prefetcher->reader->decode_queue_tail)
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prefetcher->no_readahead_until=
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prefetcher->reader->decode_queue_tail->lsn;
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returnLRQ_NEXT_AGAIN;
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}
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