pgstat: Track more detailed relation IO statistics

Commit 28e626bde00 introduced the infrastructure for tracking more detailed IO
statistics. This commit adds the actual collection of the new IO statistics
for relations and temporary relations. See aforementioned commit for goals and
high-level design.

The changes in this commit are fairly straight-forward. The bulk of the change
is to passing sufficient information to the callsites of pgstat_count_io_op().

A somewhat unsightly detail is that it currently is hard to find a better
place to count fsyncs than in md.c, whereas the other pgstat_count_io_op()
calls are in bufmgr.c/localbuf.c. As the number of fsyncs is tied to md.c
implementation details, it's not obvious there is a better answer.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20200124195226.lth52iydq2n2uilq@alap3.anarazel.de

1 files changed, 43 insertions, 15 deletions

diff --git a/src/backend/storage/buffer/freelist.c b/src/backend/storage/buffer/freelist.c
index 7dec35801cb..c690d5f15f2 100644
--- a/src/backend/storage/buffer/freelist.c
+++ b/src/backend/storage/buffer/freelist.c
@@ -15,6 +15,7 @@
  */
 #include "postgres.h"
 
+#include "pgstat.h"
 #include "port/atomics.h"
 #include "storage/buf_internals.h"
 #include "storage/bufmgr.h"
@@ -82,12 +83,6 @@ typedef struct BufferAccessStrategyData
 	int			current;
 
 	/*
-	 * True if the buffer just returned by StrategyGetBuffer had been in the
-	 * ring already.
-	 */
-	bool		current_was_in_ring;
-
-	/*
 	 * Array of buffer numbers.  InvalidBuffer (that is, zero) indicates we
 	 * have not yet selected a buffer for this ring slot.  For allocation
 	 * simplicity this is palloc'd together with the fixed fields of the
@@ -198,13 +193,15 @@ have_free_buffer(void)
  *	return the buffer with the buffer header spinlock still held.
  */
 BufferDesc *
-StrategyGetBuffer(BufferAccessStrategy strategy, uint32 *buf_state)
+StrategyGetBuffer(BufferAccessStrategy strategy, uint32 *buf_state, bool *from_ring)
 {
 	BufferDesc *buf;
 	int			bgwprocno;
 	int			trycounter;
 	uint32		local_buf_state;	/* to avoid repeated (de-)referencing */
 
+	*from_ring = false;
+
 	/*
 	 * If given a strategy object, see whether it can select a buffer. We
 	 * assume strategy objects don't need buffer_strategy_lock.
@@ -213,7 +210,10 @@ StrategyGetBuffer(BufferAccessStrategy strategy, uint32 *buf_state)
 	{
 		buf = GetBufferFromRing(strategy, buf_state);
 		if (buf != NULL)
+		{
+			*from_ring = true;
 			return buf;
+		}
 	}
 
 	/*
@@ -602,7 +602,7 @@ FreeAccessStrategy(BufferAccessStrategy strategy)
 
 /*
  * GetBufferFromRing -- returns a buffer from the ring, or NULL if the
- *		ring is empty.
+ *		ring is empty / not usable.
  *
  * The bufhdr spin lock is held on the returned buffer.
  */
@@ -625,10 +625,7 @@ GetBufferFromRing(BufferAccessStrategy strategy, uint32 *buf_state)
 	 */
 	bufnum = strategy->buffers[strategy->current];
 	if (bufnum == InvalidBuffer)
-	{
-		strategy->current_was_in_ring = false;
 		return NULL;
-	}
 
 	/*
 	 * If the buffer is pinned we cannot use it under any circumstances.
@@ -644,7 +641,6 @@ GetBufferFromRing(BufferAccessStrategy strategy, uint32 *buf_state)
 	if (BUF_STATE_GET_REFCOUNT(local_buf_state) == 0
 		&& BUF_STATE_GET_USAGECOUNT(local_buf_state) <= 1)
 	{
-		strategy->current_was_in_ring = true;
 		*buf_state = local_buf_state;
 		return buf;
 	}
@@ -654,7 +650,6 @@ GetBufferFromRing(BufferAccessStrategy strategy, uint32 *buf_state)
 	 * Tell caller to allocate a new buffer with the normal allocation
 	 * strategy.  He'll then replace this ring element via AddBufferToRing.
 	 */
-	strategy->current_was_in_ring = false;
 	return NULL;
 }
 
@@ -671,6 +666,39 @@ AddBufferToRing(BufferAccessStrategy strategy, BufferDesc *buf)
 }
 
 /*
+ * Utility function returning the IOContext of a given BufferAccessStrategy's
+ * strategy ring.
+ */
+IOContext
+IOContextForStrategy(BufferAccessStrategy strategy)
+{
+	if (!strategy)
+		return IOCONTEXT_NORMAL;
+
+	switch (strategy->btype)
+	{
+		case BAS_NORMAL:
+
+			/*
+			 * Currently, GetAccessStrategy() returns NULL for
+			 * BufferAccessStrategyType BAS_NORMAL, so this case is
+			 * unreachable.
+			 */
+			pg_unreachable();
+			return IOCONTEXT_NORMAL;
+		case BAS_BULKREAD:
+			return IOCONTEXT_BULKREAD;
+		case BAS_BULKWRITE:
+			return IOCONTEXT_BULKWRITE;
+		case BAS_VACUUM:
+			return IOCONTEXT_VACUUM;
+	}
+
+	elog(ERROR, "unrecognized BufferAccessStrategyType: %d", strategy->btype);
+	pg_unreachable();
+}
+
+/*
  * StrategyRejectBuffer -- consider rejecting a dirty buffer
  *
  * When a nondefault strategy is used, the buffer manager calls this function
@@ -682,14 +710,14 @@ AddBufferToRing(BufferAccessStrategy strategy, BufferDesc *buf)
  * if this buffer should be written and re-used.
  */
 bool
-StrategyRejectBuffer(BufferAccessStrategy strategy, BufferDesc *buf)
+StrategyRejectBuffer(BufferAccessStrategy strategy, BufferDesc *buf, bool from_ring)
 {
 	/* We only do this in bulkread mode */
 	if (strategy->btype != BAS_BULKREAD)
 		return false;
 
 	/* Don't muck with behavior of normal buffer-replacement strategy */
-	if (!strategy->current_was_in_ring ||
+	if (!from_ring ||
 		strategy->buffers[strategy->current] != BufferDescriptorGetBuffer(buf))
 		return false;