Cache smgrnblocks() results in recovery.

Avoid repeatedly calling lseek(SEEK_END) during recovery by caching
the size of each fork.  For now, we can't use the same technique in
other processes, because we lack a shared invalidation mechanism.

Do this by generalizing the pre-existing caching used by FSM and VM
to support all forks.

Discussion: https://postgr.es/m/CAEepm%3D3SSw-Ty1DFcK%3D1rU-K6GSzYzfdD4d%2BZwapdN7dTa6%3DnQ%40mail.gmail.com

Author: macdice

contrib/pg_visibility/pg_visibility.c

@@ -392,7 +392,7 @@ pg_truncate_visibility_map(PG_FUNCTION_ARGS)
 	check_relation_relkind(rel);
 
 	RelationOpenSmgr(rel);
-	rel->rd_smgr->smgr_vm_nblocks = InvalidBlockNumber;
+	rel->rd_smgr->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] = InvalidBlockNumber;
 
 	block = visibilitymap_prepare_truncate(rel, 0);
 	if (BlockNumberIsValid(block))

src/backend/access/heap/visibilitymap.c

@@ -561,17 +561,16 @@ vm_readbuf(Relation rel, BlockNumber blkno, bool extend)
 	 * If we haven't cached the size of the visibility map fork yet, check it
 	 * first.
 	 */
-	if (rel->rd_smgr->smgr_vm_nblocks == InvalidBlockNumber)
+	if (rel->rd_smgr->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] == InvalidBlockNumber)
 	{
 		if (smgrexists(rel->rd_smgr, VISIBILITYMAP_FORKNUM))
-			rel->rd_smgr->smgr_vm_nblocks = smgrnblocks(rel->rd_smgr,
-														VISIBILITYMAP_FORKNUM);
+			smgrnblocks(rel->rd_smgr, VISIBILITYMAP_FORKNUM);
 		else
-			rel->rd_smgr->smgr_vm_nblocks = 0;
+			rel->rd_smgr->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] = 0;
 	}
 
 	/* Handle requests beyond EOF */
-	if (blkno >= rel->rd_smgr->smgr_vm_nblocks)
+	if (blkno >= rel->rd_smgr->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM])
 	{
 		if (extend)
 			vm_extend(rel, blkno + 1);
@@ -641,11 +640,13 @@ vm_extend(Relation rel, BlockNumber vm_nblocks)
 	 * Create the file first if it doesn't exist.  If smgr_vm_nblocks is
 	 * positive then it must exist, no need for an smgrexists call.
 	 */
-	if ((rel->rd_smgr->smgr_vm_nblocks == 0 ||
-		 rel->rd_smgr->smgr_vm_nblocks == InvalidBlockNumber) &&
+	if ((rel->rd_smgr->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] == 0 ||
+		 rel->rd_smgr->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] == InvalidBlockNumber) &&
 		!smgrexists(rel->rd_smgr, VISIBILITYMAP_FORKNUM))
 		smgrcreate(rel->rd_smgr, VISIBILITYMAP_FORKNUM, false);
 
+	/* Invalidate cache so that smgrnblocks() asks the kernel. */
+	rel->rd_smgr->smgr_cached_nblocks[VISIBILITYMAP_FORKNUM] = InvalidBlockNumber;
 	vm_nblocks_now = smgrnblocks(rel->rd_smgr, VISIBILITYMAP_FORKNUM);
 
 	/* Now extend the file */
@@ -667,8 +668,5 @@ vm_extend(Relation rel, BlockNumber vm_nblocks)
 	 */
 	CacheInvalidateSmgr(rel->rd_smgr->smgr_rnode);
 
-	/* Update local cache with the up-to-date size */
-	rel->rd_smgr->smgr_vm_nblocks = vm_nblocks_now;
-
 	UnlockRelationForExtension(rel, ExclusiveLock);
 }

src/backend/catalog/storage.c

@@ -290,8 +290,8 @@ RelationTruncate(Relation rel, BlockNumber nblocks)
 	 * Make sure smgr_targblock etc aren't pointing somewhere past new end
 	 */
 	rel->rd_smgr->smgr_targblock = InvalidBlockNumber;
-	rel->rd_smgr->smgr_fsm_nblocks = InvalidBlockNumber;
-	rel->rd_smgr->smgr_vm_nblocks = InvalidBlockNumber;
+	for (int i = 0; i <= MAX_FORKNUM; ++i)
+		rel->rd_smgr->smgr_cached_nblocks[i] = InvalidBlockNumber;
 
 	/* Prepare for truncation of MAIN fork of the relation */
 	forks[nforks] = MAIN_FORKNUM;

src/backend/storage/freespace/freespace.c

@@ -541,18 +541,19 @@ fsm_readbuf(Relation rel, FSMAddress addr, bool extend)
 	 * value might be stale.  (We send smgr inval messages on truncation, but
 	 * not on extension.)
 	 */
-	if (rel->rd_smgr->smgr_fsm_nblocks == InvalidBlockNumber ||
-		blkno >= rel->rd_smgr->smgr_fsm_nblocks)
+	if (rel->rd_smgr->smgr_cached_nblocks[FSM_FORKNUM] == InvalidBlockNumber ||
+		blkno >= rel->rd_smgr->smgr_cached_nblocks[FSM_FORKNUM])
 	{
+		/* Invalidate the cache so smgrnblocks asks the kernel. */
+		rel->rd_smgr->smgr_cached_nblocks[FSM_FORKNUM] = InvalidBlockNumber;
 		if (smgrexists(rel->rd_smgr, FSM_FORKNUM))
-			rel->rd_smgr->smgr_fsm_nblocks = smgrnblocks(rel->rd_smgr,
-														 FSM_FORKNUM);
+			smgrnblocks(rel->rd_smgr, FSM_FORKNUM);
 		else
-			rel->rd_smgr->smgr_fsm_nblocks = 0;
+			rel->rd_smgr->smgr_cached_nblocks[FSM_FORKNUM] = 0;
 	}
 
 	/* Handle requests beyond EOF */
-	if (blkno >= rel->rd_smgr->smgr_fsm_nblocks)
+	if (blkno >= rel->rd_smgr->smgr_cached_nblocks[FSM_FORKNUM])
 	{
 		if (extend)
 			fsm_extend(rel, blkno + 1);
@@ -621,14 +622,17 @@ fsm_extend(Relation rel, BlockNumber fsm_nblocks)
 	RelationOpenSmgr(rel);
 
 	/*
-	 * Create the FSM file first if it doesn't exist.  If smgr_fsm_nblocks is
-	 * positive then it must exist, no need for an smgrexists call.
+	 * Create the FSM file first if it doesn't exist.  If
+	 * smgr_cached_nblocks[FSM_FORKNUM] is positive then it must exist, no
+	 * need for an smgrexists call.
 	 */
-	if ((rel->rd_smgr->smgr_fsm_nblocks == 0 ||
-		 rel->rd_smgr->smgr_fsm_nblocks == InvalidBlockNumber) &&
+	if ((rel->rd_smgr->smgr_cached_nblocks[FSM_FORKNUM] == 0 ||
+		 rel->rd_smgr->smgr_cached_nblocks[FSM_FORKNUM] == InvalidBlockNumber) &&
 		!smgrexists(rel->rd_smgr, FSM_FORKNUM))
 		smgrcreate(rel->rd_smgr, FSM_FORKNUM, false);
 
+	/* Invalidate cache so that smgrnblocks() asks the kernel. */
+	rel->rd_smgr->smgr_cached_nblocks[FSM_FORKNUM] = InvalidBlockNumber;
 	fsm_nblocks_now = smgrnblocks(rel->rd_smgr, FSM_FORKNUM);
 
 	while (fsm_nblocks_now < fsm_nblocks)
@@ -640,9 +644,6 @@ fsm_extend(Relation rel, BlockNumber fsm_nblocks)
 		fsm_nblocks_now++;
 	}
 
-	/* Update local cache with the up-to-date size */
-	rel->rd_smgr->smgr_fsm_nblocks = fsm_nblocks_now;
-
 	UnlockRelationForExtension(rel, ExclusiveLock);
 }
 

src/backend/storage/smgr/smgr.c

@@ -17,6 +17,7 @@
  */
 #include "postgres.h"
 
+#include "access/xlog.h"
 #include "lib/ilist.h"
 #include "storage/bufmgr.h"
 #include "storage/ipc.h"
@@ -174,8 +175,8 @@ smgropen(RelFileNode rnode, BackendId backend)
 		/* hash_search already filled in the lookup key */
 		reln->smgr_owner = NULL;
 		reln->smgr_targblock = InvalidBlockNumber;
-		reln->smgr_fsm_nblocks = InvalidBlockNumber;
-		reln->smgr_vm_nblocks = InvalidBlockNumber;
+		for (int i = 0; i <= MAX_FORKNUM; ++i)
+			reln->smgr_cached_nblocks[i] = InvalidBlockNumber;
 		reln->smgr_which = 0;	/* we only have md.c at present */
 
 		/* implementation-specific initialization */
@@ -464,6 +465,16 @@ smgrextend(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 {
 	smgrsw[reln->smgr_which].smgr_extend(reln, forknum, blocknum,
 										 buffer, skipFsync);
+
+	/*
+	 * Normally we expect this to increase nblocks by one, but if the cached
+	 * value isn't as expected, just invalidate it so the next call asks the
+	 * kernel.
+	 */
+	if (reln->smgr_cached_nblocks[forknum] == blocknum)
+		reln->smgr_cached_nblocks[forknum] = blocknum + 1;
+	else
+		reln->smgr_cached_nblocks[forknum] = InvalidBlockNumber;
 }
 
 /*
@@ -537,7 +548,20 @@ smgrwriteback(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 BlockNumber
 smgrnblocks(SMgrRelation reln, ForkNumber forknum)
 {
-	return smgrsw[reln->smgr_which].smgr_nblocks(reln, forknum);
+	BlockNumber result;
+
+	/*
+	 * For now, we only use cached values in recovery due to lack of a shared
+	 * invalidation mechanism for changes in file size.
+	 */
+	if (InRecovery && reln->smgr_cached_nblocks[forknum] != InvalidBlockNumber)
+		return reln->smgr_cached_nblocks[forknum];
+
+	result = smgrsw[reln->smgr_which].smgr_nblocks(reln, forknum);
+
+	reln->smgr_cached_nblocks[forknum] = result;
+
+	return result;
 }
 
 /*
@@ -576,20 +600,19 @@ smgrtruncate(SMgrRelation reln, ForkNumber *forknum, int nforks, BlockNumber *nb
 	/* Do the truncation */
 	for (i = 0; i < nforks; i++)
 	{
+		/* Make the cached size is invalid if we encounter an error. */
+		reln->smgr_cached_nblocks[forknum[i]] = InvalidBlockNumber;
+
 		smgrsw[reln->smgr_which].smgr_truncate(reln, forknum[i], nblocks[i]);
 
 		/*
-		 * We might as well update the local smgr_fsm_nblocks and
-		 * smgr_vm_nblocks settings. The smgr cache inval message that this
-		 * function sent will cause other backends to invalidate their copies
-		 * of smgr_fsm_nblocks and smgr_vm_nblocks, and these ones too at the
-		 * next command boundary. But these ensure they aren't outright wrong
-		 * until then.
+		 * We might as well update the local smgr_cached_nblocks values. The
+		 * smgr cache inval message that this function sent will cause other
+		 * backends to invalidate their copies of smgr_fsm_nblocks and
+		 * smgr_vm_nblocks, and these ones too at the next command boundary.
+		 * But these ensure they aren't outright wrong until then.
 		 */
-		if (forknum[i] == FSM_FORKNUM)
-			reln->smgr_fsm_nblocks = nblocks[i];
-		if (forknum[i] == VISIBILITYMAP_FORKNUM)
-			reln->smgr_vm_nblocks = nblocks[i];
+		reln->smgr_cached_nblocks[forknum[i]] = nblocks[i];
 	}
 }
 

src/include/storage/smgr.h

@@ -45,15 +45,13 @@ typedef struct SMgrRelationData
 	struct SMgrRelationData **smgr_owner;
 
 	/*
-	 * These next three fields are not actually used or manipulated by smgr,
-	 * except that they are reset to InvalidBlockNumber upon a cache flush
-	 * event (in particular, upon truncation of the relation).  Higher levels
-	 * store cached state here so that it will be reset when truncation
-	 * happens.  In all three cases, InvalidBlockNumber means "unknown".
+	 * The following fields are reset to InvalidBlockNumber upon a cache flush
+	 * event, and hold the last known size for each fork.  This information is
+	 * currently only reliable during recovery, since there is no cache
+	 * invalidation for fork extension.
 	 */
 	BlockNumber smgr_targblock; /* current insertion target block */
-	BlockNumber smgr_fsm_nblocks;	/* last known size of fsm fork */
-	BlockNumber smgr_vm_nblocks;	/* last known size of vm fork */
+	BlockNumber smgr_cached_nblocks[MAX_FORKNUM + 1];	/* last known size */
 
 	/* additional public fields may someday exist here */