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Improve replication lag interpolation after idle period 

After sitting idle and fully replayed for a while and then encountering
a new burst of WAL activity, we interpolate between an ancient sample and the
not-yet-reached one for the new traffic. That produced a corner case report
of lag after receiving first new reply from standby, which might sometimes
be a large spike.

Correct this by resetting last_read time and handle that new case.

Author: Thomas Munro
This commit is contained in:
Simon Riggs 2017-06-23 18:58:46 +01:00
parent a79122b061
commit 9ea3c64124
1 changed files with 25 additions and 4 deletions
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29
src/backend/replication/walsender.c
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@ -3443,6 +3443,16 @@ LagTrackerRead(int head, XLogRecPtr lsn, TimestampTz now)
(LagTracker.read_heads[head] + 1) % LAG_TRACKER_BUFFER_SIZE;
}
/*
* If the lag tracker is empty, that means the standby has processed
* everything we've ever sent so we should now clear 'last_read'. If we
* didn't do that, we'd risk using a stale and irrelevant sample for
* interpolation at the beginning of the next burst of WAL after a period
* of idleness.
*/
if (LagTracker.read_heads[head] == LagTracker.write_head)
LagTracker.last_read[head].time = 0;
if (time > now)
{
/* If the clock somehow went backwards, treat as not found. */
@ -3459,9 +3469,14 @@ LagTrackerRead(int head, XLogRecPtr lsn, TimestampTz now)
* eventually start moving again and cross one of our samples before
* we can show the lag increasing.
*/
if (LagTracker.read_heads[head] != LagTracker.write_head &&
LagTracker.last_read[head].time != 0)
if (LagTracker.read_heads[head] == LagTracker.write_head)
{
/* There are no future samples, so we can't interpolate. */
return -1;
}
else if (LagTracker.last_read[head].time != 0)
{
/* We can interpolate between last_read and the next sample. */
double fraction;
WalTimeSample prev = LagTracker.last_read[head];
WalTimeSample next = LagTracker.buffer[LagTracker.read_heads[head]];
@ -3494,8 +3509,14 @@ LagTrackerRead(int head, XLogRecPtr lsn, TimestampTz now)
}
else
{
/* Couldn't interpolate due to lack of data. */
return -1;
/*
* We have only a future sample, implying that we were entirely
* caught up but and now there is a new burst of WAL and the
* standby hasn't processed the first sample yet. Until the
* standby reaches the future sample the best we can do is report
* the hypothetical lag if that sample were to be replayed now.
*/
time = LagTracker.buffer[LagTracker.read_heads[head]].time;
}
}