Add GUC to enable compression of full page images stored in WAL.

When newly-added GUC parameter, wal_compression, is on, the PostgreSQL server
compresses a full page image written to WAL when full_page_writes is on or
during a base backup. A compressed page image will be decompressed during WAL
replay. Turning this parameter on can reduce the WAL volume without increasing
the risk of unrecoverable data corruption, but at the cost of some extra CPU
spent on the compression during WAL logging and on the decompression during
WAL replay.

This commit changes the WAL format (so bumping WAL version number) so that
the one-byte flag indicating whether a full page image is compressed or not is
included in its header information. This means that the commit increases the
WAL volume one-byte per a full page image even if WAL compression is not used
at all. We can save that one-byte by borrowing one-bit from the existing field
like hole_offset in the header and using it as the flag, for example. But which
would reduce the code readability and the extensibility of the feature.
Per discussion, it's not worth paying those prices to save only one-byte, so we
decided to add the one-byte flag to the header.

This commit doesn't introduce any new compression algorithm like lz4.
Currently a full page image is compressed using the existing PGLZ algorithm.
Per discussion, we decided to use it at least in the first version of the
feature because there were no performance reports showing that its compression
ratio is unacceptably lower than that of other algorithm. Of course,
in the future, it's worth considering the support of other compression
algorithm for the better compression.

Rahila Syed and Michael Paquier, reviewed in various versions by myself,
Andres Freund, Robert Haas, Abhijit Menon-Sen and many others.

src/include/access/xlogrecord.h

@@ -100,18 +100,55 @@ typedef struct XLogRecordBlockHeader
  *
  * As a trivial form of data compression, the XLOG code is aware that
  * PG data pages usually contain an unused "hole" in the middle, which
- * contains only zero bytes.  If hole_length > 0 then we have removed
+ * contains only zero bytes.  If the length of "hole" > 0 then we have removed
  * such a "hole" from the stored data (and it's not counted in the
  * XLOG record's CRC, either).  Hence, the amount of block data actually
- * present is BLCKSZ - hole_length bytes.
+ * present is BLCKSZ - the length of "hole" bytes.
+ *
+ * When wal_compression is enabled, a full page image which "hole" was
+ * removed is additionally compressed using PGLZ compression algorithm.
+ * This can reduce the WAL volume, but at some extra cost of CPU spent
+ * on the compression during WAL logging. In this case, since the "hole"
+ * length cannot be calculated by subtracting the number of page image bytes
+ * from BLCKSZ, basically it needs to be stored as an extra information.
+ * But when no "hole" exists, we can assume that the "hole" length is zero
+ * and no such an extra information needs to be stored. Note that
+ * the original version of page image is stored in WAL instead of the
+ * compressed one if the number of bytes saved by compression is less than
+ * the length of extra information. Hence, when a page image is successfully
+ * compressed, the amount of block data actually present is less than
+ * BLCKSZ - the length of "hole" bytes - the length of extra information.
  */
 typedef struct XLogRecordBlockImageHeader
 {
-	uint16		hole_offset;	/* number of bytes before "hole" */
-	uint16		hole_length;	/* number of bytes in "hole" */
+	uint16	length;	/* number of page image bytes */
+	uint16	hole_offset;	/* number of bytes before "hole" */
+	uint8		bimg_info;	/* flag bits, see below */
+
+	/*
+	 * If BKPIMAGE_HAS_HOLE and BKPIMAGE_IS_COMPRESSED,
+	 * an XLogRecordBlockCompressHeader struct follows.
+	 */
 } XLogRecordBlockImageHeader;
 
-#define SizeOfXLogRecordBlockImageHeader sizeof(XLogRecordBlockImageHeader)
+#define SizeOfXLogRecordBlockImageHeader	\
+	(offsetof(XLogRecordBlockImageHeader, bimg_info) + sizeof(uint8))
+
+/* Information stored in bimg_info */
+#define BKPIMAGE_HAS_HOLE		0x01	/* page image has "hole" */
+#define BKPIMAGE_IS_COMPRESSED		0x02	/* page image is compressed */
+
+/*
+ * Extra header information used when page image has "hole" and
+ * is compressed.
+ */
+typedef struct XLogRecordBlockCompressHeader
+{
+	uint16	hole_length;	/* number of bytes in "hole" */
+} XLogRecordBlockCompressHeader;
+
+#define SizeOfXLogRecordBlockCompressHeader \
+	sizeof(XLogRecordBlockCompressHeader)
 
 /*
  * Maximum size of the header for a block reference. This is used to size a
@@ -120,6 +157,7 @@ typedef struct XLogRecordBlockImageHeader
 #define MaxSizeOfXLogRecordBlockHeader \
 	(SizeOfXLogRecordBlockHeader + \
 	 SizeOfXLogRecordBlockImageHeader + \
+	 SizeOfXLogRecordBlockCompressHeader + \
 	 sizeof(RelFileNode) + \
 	 sizeof(BlockNumber))