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Remove the "snapshot too old" feature.

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Remove the old_snapshot_threshold setting and mechanism for producing
the error "snapshot too old", originally added by commit 848ef42b.
Unfortunately it had a number of known problems in terms of correctness
and performance, mostly reported by Andres in the course of his work on
snapshot scalability.  We agreed to remove it, after a long period
without an active plan to fix it.

This is certainly a desirable feature, and someone might propose a new
or improved implementation in the future.

Reported-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CACG%3DezYV%2BEvO135fLRdVn-ZusfVsTY6cH1OZqWtezuEYH6ciQA%40mail.gmail.com
Discussion: https://postgr.es/m/20200401064008.qob7bfnnbu4w5cw4%40alap3.anarazel.de
Discussion: https://postgr.es/m/CA%2BTgmoY%3Daqf0zjTD%2B3dUWYkgMiNDegDLFjo%2B6ze%3DWtpik%2B3XqA%40mail.gmail.com
This commit is contained in:
Thomas Munro
2023-09-05 18:26:12 +12:00
parent aa0d350456
commit f691f5b80a
50 changed files with 21 additions and 1425 deletions
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36
src/backend/storage/ipc/procarray.c
View File

@@ -2066,34 +2066,6 @@ GetMaxSnapshotSubxidCount(void)
return TOTAL_MAX_CACHED_SUBXIDS;
}
/*
* Initialize old_snapshot_threshold specific parts of a newly build snapshot.
*/
static void
GetSnapshotDataInitOldSnapshot(Snapshot snapshot)
{
if (!OldSnapshotThresholdActive())
{
/*
* If not using "snapshot too old" feature, fill related fields with
* dummy values that don't require any locking.
*/
snapshot->lsn = InvalidXLogRecPtr;
snapshot->whenTaken = 0;
}
else
{
/*
* Capture the current time and WAL stream location in case this
* snapshot becomes old enough to need to fall back on the special
* "old snapshot" logic.
*/
snapshot->lsn = GetXLogInsertRecPtr();
snapshot->whenTaken = GetSnapshotCurrentTimestamp();
MaintainOldSnapshotTimeMapping(snapshot->whenTaken, snapshot->xmin);
}
}
/*
* Helper function for GetSnapshotData() that checks if the bulk of the
* visibility information in the snapshot is still valid. If so, it updates
@@ -2147,8 +2119,8 @@ GetSnapshotDataReuse(Snapshot snapshot)
snapshot->active_count = 0;
snapshot->regd_count = 0;
snapshot->copied = false;
GetSnapshotDataInitOldSnapshot(snapshot);
snapshot->lsn = InvalidXLogRecPtr;
snapshot->whenTaken = 0;
return true;
}
@@ -2529,8 +2501,8 @@ GetSnapshotData(Snapshot snapshot)
snapshot->active_count = 0;
snapshot->regd_count = 0;
snapshot->copied = false;
GetSnapshotDataInitOldSnapshot(snapshot);
snapshot->lsn = InvalidXLogRecPtr;
snapshot->whenTaken = 0;
return snapshot;
}