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

1 files changed, 36 insertions, 13 deletions

diff --git a/src/backend/storage/smgr/smgr.c b/src/backend/storage/smgr/smgr.c
index 7d667c6586f..dcc09df0c77 100644
--- a/src/backend/storage/smgr/smgr.c
+++ b/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];
 	}
 }