From b3f853589e55f78878923434810b0c6fb82c230e Mon Sep 17 00:00:00 2001 From: Alexander Korotkov Date: Sat, 24 May 2025 13:09:16 +0300 Subject: [PATCH v5 1/3] Keep WAL segments by the flushed value of the slot's restart LSN The patch fixes the issue with the unexpected removal of old WAL segments after checkpoint, followed by an immediate restart. The issue occurs when a slot is advanced after the start of the checkpoint and before old WAL segments are removed at the end of the checkpoint. The idea of the patch is to get the minimal restart_lsn at the beginning of checkpoint (or restart point) creation and use this value when calculating the oldest LSN for WAL segments removal at the end of checkpoint. This idea was proposed by Tomas Vondra in the discussion. Discussion: https://postgr.es/m/flat/1d12d2-67235980-35-19a406a0%4063439497 Author: Vitaly Davydov Reviewed-by: Tomas Vondra Reviewed-by: Alexander Korotkov Backpatch-through: 13 --- src/backend/access/transam/xlog.c | 55 ++++++++++++++++++++++++++----- 1 file changed, 47 insertions(+), 8 deletions(-) diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c index 1914859b2ee..a0e589e9c4b 100644 --- a/src/backend/access/transam/xlog.c +++ b/src/backend/access/transam/xlog.c @@ -677,7 +677,8 @@ static XLogRecPtr CreateOverwriteContrecordRecord(XLogRecPtr aborted_lsn, XLogRecPtr pagePtr, TimeLineID newTLI); static void CheckPointGuts(XLogRecPtr checkPointRedo, int flags); -static void KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo); +static void KeepLogSeg(XLogRecPtr recptr, XLogRecPtr slotsMinLSN, + XLogSegNo *logSegNo); static XLogRecPtr XLogGetReplicationSlotMinimumLSN(void); static void AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, @@ -7087,6 +7088,7 @@ CreateCheckPoint(int flags) VirtualTransactionId *vxids; int nvxids; int oldXLogAllowed = 0; + XLogRecPtr slotsMinReqLSN; /* * An end-of-recovery checkpoint is really a shutdown checkpoint, just @@ -7315,6 +7317,15 @@ CreateCheckPoint(int flags) */ END_CRIT_SECTION(); + /* + * Get the current minimum LSN to be used later in the WAL segment + * cleanup. We may clean up only WAL segments, which are not needed + * according to synchronized LSNs of replication slots. The slot's LSN + * might be advanced concurrently, so we call this before + * CheckPointReplicationSlots() synchronizes replication slots. + */ + slotsMinReqLSN = XLogGetReplicationSlotMinimumLSN(); + /* * In some cases there are groups of actions that must all occur on one * side or the other of a checkpoint record. Before flushing the @@ -7503,17 +7514,25 @@ CreateCheckPoint(int flags) * prevent the disk holding the xlog from growing full. */ XLByteToSeg(RedoRecPtr, _logSegNo, wal_segment_size); - KeepLogSeg(recptr, &_logSegNo); + KeepLogSeg(recptr, slotsMinReqLSN, &_logSegNo); if (InvalidateObsoleteReplicationSlots(RS_INVAL_WAL_REMOVED | RS_INVAL_IDLE_TIMEOUT, _logSegNo, InvalidOid, InvalidTransactionId)) { + /* + * Recalculate the current minimum LSN to be used in the WAL segment + * cleanup. Then, we must synchronize the replication slots again in + * order to make this LSN safe to use. + */ + slotsMinReqLSN = XLogGetReplicationSlotMinimumLSN(); + CheckPointReplicationSlots(shutdown); + /* * Some slots have been invalidated; recalculate the old-segment * horizon, starting again from RedoRecPtr. */ XLByteToSeg(RedoRecPtr, _logSegNo, wal_segment_size); - KeepLogSeg(recptr, &_logSegNo); + KeepLogSeg(recptr, slotsMinReqLSN, &_logSegNo); } _logSegNo--; RemoveOldXlogFiles(_logSegNo, RedoRecPtr, recptr, @@ -7788,6 +7807,7 @@ CreateRestartPoint(int flags) XLogRecPtr endptr; XLogSegNo _logSegNo; TimestampTz xtime; + XLogRecPtr slotsMinReqLSN; /* Concurrent checkpoint/restartpoint cannot happen */ Assert(!IsUnderPostmaster || MyBackendType == B_CHECKPOINTER); @@ -7870,6 +7890,15 @@ CreateRestartPoint(int flags) MemSet(&CheckpointStats, 0, sizeof(CheckpointStats)); CheckpointStats.ckpt_start_t = GetCurrentTimestamp(); + /* + * Get the current minimum LSN to be used later in the WAL segment + * cleanup. We may clean up only WAL segments, which are not needed + * according to synchronized LSNs of replication slots. The slot's LSN + * might be advanced concurrently, so we call this before + * CheckPointReplicationSlots() synchronizes replication slots. + */ + slotsMinReqLSN = XLogGetReplicationSlotMinimumLSN(); + if (log_checkpoints) LogCheckpointStart(flags, true); @@ -7958,17 +7987,25 @@ CreateRestartPoint(int flags) receivePtr = GetWalRcvFlushRecPtr(NULL, NULL); replayPtr = GetXLogReplayRecPtr(&replayTLI); endptr = (receivePtr < replayPtr) ? replayPtr : receivePtr; - KeepLogSeg(endptr, &_logSegNo); + KeepLogSeg(endptr, slotsMinReqLSN, &_logSegNo); if (InvalidateObsoleteReplicationSlots(RS_INVAL_WAL_REMOVED | RS_INVAL_IDLE_TIMEOUT, _logSegNo, InvalidOid, InvalidTransactionId)) { + /* + * Recalculate the current minimum LSN to be used in the WAL segment + * cleanup. Then, we must synchronize the replication slots again in + * order to make this LSN safe to use. + */ + slotsMinReqLSN = XLogGetReplicationSlotMinimumLSN(); + CheckPointReplicationSlots(flags & CHECKPOINT_IS_SHUTDOWN); + /* * Some slots have been invalidated; recalculate the old-segment * horizon, starting again from RedoRecPtr. */ XLByteToSeg(RedoRecPtr, _logSegNo, wal_segment_size); - KeepLogSeg(endptr, &_logSegNo); + KeepLogSeg(endptr, slotsMinReqLSN, &_logSegNo); } _logSegNo--; @@ -8063,6 +8100,7 @@ GetWALAvailability(XLogRecPtr targetLSN) XLogSegNo oldestSegMaxWalSize; /* oldest segid kept by max_wal_size */ XLogSegNo oldestSlotSeg; /* oldest segid kept by slot */ uint64 keepSegs; + XLogRecPtr slotsMinReqLSN; /* * slot does not reserve WAL. Either deactivated, or has never been active @@ -8076,8 +8114,9 @@ GetWALAvailability(XLogRecPtr targetLSN) * oldestSlotSeg to the current segment. */ currpos = GetXLogWriteRecPtr(); + slotsMinReqLSN = XLogGetReplicationSlotMinimumLSN(); XLByteToSeg(currpos, oldestSlotSeg, wal_segment_size); - KeepLogSeg(currpos, &oldestSlotSeg); + KeepLogSeg(currpos, slotsMinReqLSN, &oldestSlotSeg); /* * Find the oldest extant segment file. We get 1 until checkpoint removes @@ -8138,7 +8177,7 @@ GetWALAvailability(XLogRecPtr targetLSN) * invalidation is optionally done here, instead. */ static void -KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo) +KeepLogSeg(XLogRecPtr recptr, XLogRecPtr slotsMinReqLSN, XLogSegNo *logSegNo) { XLogSegNo currSegNo; XLogSegNo segno; @@ -8151,7 +8190,7 @@ KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo) * Calculate how many segments are kept by slots first, adjusting for * max_slot_wal_keep_size. */ - keep = XLogGetReplicationSlotMinimumLSN(); + keep = slotsMinReqLSN; if (keep != InvalidXLogRecPtr && keep < recptr) { XLByteToSeg(keep, segno, wal_segment_size); -- 2.39.5 (Apple Git-154)