Introduce PG_IO_ALIGN_SIZE and align all I/O buffers.

In order to have the option to use O_DIRECT/FILE_FLAG_NO_BUFFERING in a
later commit, we need the addresses of user space buffers to be well
aligned.  The exact requirements vary by OS and file system (typically
sectors and/or memory pages).  The address alignment size is set to
4096, which is enough for currently known systems: it matches modern
sectors and common memory page size.  There is no standard governing
O_DIRECT's requirements so we might eventually have to reconsider this
with more information from the field or future systems.

Aligning I/O buffers on memory pages is also known to improve regular
buffered I/O performance.

Three classes of I/O buffers for regular data pages are adjusted:
(1) Heap buffers are now allocated with the new palloc_aligned() or
MemoryContextAllocAligned() functions introduced by commit 439f6175.
(2) Stack buffers now use a new struct PGIOAlignedBlock to respect
PG_IO_ALIGN_SIZE, if possible with this compiler.  (3) The buffer
pool is also aligned in shared memory.

WAL buffers were already aligned on XLOG_BLCKSZ.  It's possible for
XLOG_BLCKSZ to be configured smaller than PG_IO_ALIGNED_SIZE and thus
for O_DIRECT WAL writes to fail to be well aligned, but that's a
pre-existing condition and will be addressed by a later commit.

BufFiles are not yet addressed (there's no current plan to use O_DIRECT
for those, but they could potentially get some incidental speedup even
in plain buffered I/O operations through better alignment).

If we can't align stack objects suitably using the compiler extensions
we know about, we disable the use of O_DIRECT by setting PG_O_DIRECT to
0.  This avoids the need to consider systems that have O_DIRECT but
can't align stack objects the way we want; such systems could in theory
be supported with more work but we don't currently know of any such
machines, so it's easier to pretend there is no O_DIRECT support
instead.  That's an existing and tested class of system.

Add assertions that all buffers passed into smgrread(), smgrwrite() and
smgrextend() are correctly aligned, unless PG_O_DIRECT is 0 (= stack
alignment tricks may be unavailable) or the block size has been set too
small to allow arrays of buffers to be all aligned.

Author: Thomas Munro <thomas.munro@gmail.com>
Author: Andres Freund <andres@anarazel.de>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/CA+hUKGK1X532hYqJ_MzFWt0n1zt8trz980D79WbjwnT-yYLZpg@mail.gmail.com

src/backend/access/transam/generic_xlog.c

@@ -58,14 +58,17 @@ typedef struct
 	char		delta[MAX_DELTA_SIZE];	/* delta between page images */
 } PageData;
 
-/* State of generic xlog record construction */
+/*
+ * State of generic xlog record construction.  Must be allocated at an I/O
+ * aligned address.
+ */
 struct GenericXLogState
 {
+	/* Page images (properly aligned, must be first) */
+	PGIOAlignedBlock images[MAX_GENERIC_XLOG_PAGES];
 	/* Info about each page, see above */
 	PageData	pages[MAX_GENERIC_XLOG_PAGES];
 	bool		isLogged;
-	/* Page images (properly aligned) */
-	PGAlignedBlock images[MAX_GENERIC_XLOG_PAGES];
 };
 
 static void writeFragment(PageData *pageData, OffsetNumber offset,
@@ -269,7 +272,9 @@ GenericXLogStart(Relation relation)
 	GenericXLogState *state;
 	int			i;
 
-	state = (GenericXLogState *) palloc(sizeof(GenericXLogState));
+	state = (GenericXLogState *) palloc_aligned(sizeof(GenericXLogState),
+												PG_IO_ALIGN_SIZE,
+												0);
 	state->isLogged = RelationNeedsWAL(relation);
 
 	for (i = 0; i < MAX_GENERIC_XLOG_PAGES; i++)

src/backend/access/transam/xlog.c

@@ -4506,7 +4506,7 @@ XLOGShmemSize(void)
 	/* xlblocks array */
 	size = add_size(size, mul_size(sizeof(XLogRecPtr), XLOGbuffers));
 	/* extra alignment padding for XLOG I/O buffers */
-	size = add_size(size, XLOG_BLCKSZ);
+	size = add_size(size, Max(XLOG_BLCKSZ, PG_IO_ALIGN_SIZE));
 	/* and the buffers themselves */
 	size = add_size(size, mul_size(XLOG_BLCKSZ, XLOGbuffers));