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Commit9ea3c64

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Improve replication lag interpolation after idle period
After sitting idle and fully replayed for a while and then encounteringa new burst of WAL activity, we interpolate between an ancient sample and thenot-yet-reached one for the new traffic. That produced a corner case reportof lag after receiving first new reply from standby, which might sometimesbe a large spike.Correct this by resetting last_read time and handle that new case.Author: Thomas Munro
1 parenta79122b commit9ea3c64

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-4
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1 file changed

+25
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‎src/backend/replication/walsender.c

Lines changed: 25 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -3443,6 +3443,16 @@ LagTrackerRead(int head, XLogRecPtr lsn, TimestampTz now)
34433443
(LagTracker.read_heads[head]+1) %LAG_TRACKER_BUFFER_SIZE;
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}
34453445

3446+
/*
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* If the lag tracker is empty, that means the standby has processed
3448+
* everything we've ever sent so we should now clear 'last_read'. If we
3449+
* didn't do that, we'd risk using a stale and irrelevant sample for
3450+
* interpolation at the beginning of the next burst of WAL after a period
3451+
* of idleness.
3452+
*/
3453+
if (LagTracker.read_heads[head]==LagTracker.write_head)
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LagTracker.last_read[head].time=0;
3455+
34463456
if (time>now)
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{
34483458
/* If the clock somehow went backwards, treat as not found. */
@@ -3459,9 +3469,14 @@ LagTrackerRead(int head, XLogRecPtr lsn, TimestampTz now)
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* eventually start moving again and cross one of our samples before
34603470
* we can show the lag increasing.
34613471
*/
3462-
if (LagTracker.read_heads[head]!=LagTracker.write_head&&
3463-
LagTracker.last_read[head].time!=0)
3472+
if (LagTracker.read_heads[head]==LagTracker.write_head)
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{
3474+
/* There are no future samples, so we can't interpolate. */
3475+
return-1;
3476+
}
3477+
elseif (LagTracker.last_read[head].time!=0)
3478+
{
3479+
/* We can interpolate between last_read and the next sample. */
34653480
doublefraction;
34663481
WalTimeSampleprev=LagTracker.last_read[head];
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WalTimeSamplenext=LagTracker.buffer[LagTracker.read_heads[head]];
@@ -3494,8 +3509,14 @@ LagTrackerRead(int head, XLogRecPtr lsn, TimestampTz now)
34943509
}
34953510
else
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{
3497-
/* Couldn't interpolate due to lack of data. */
3498-
return-1;
3512+
/*
3513+
* We have only a future sample, implying that we were entirely
3514+
* caught up but and now there is a new burst of WAL and the
3515+
* standby hasn't processed the first sample yet. Until the
3516+
* standby reaches the future sample the best we can do is report
3517+
* the hypothetical lag if that sample were to be replayed now.
3518+
*/
3519+
time=LagTracker.buffer[LagTracker.read_heads[head]].time;
34993520
}
35003521
}
35013522

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