Provide vectored variants of smgrread() and smgrwrite().

smgrreadv() and smgrwritev() and their md.c implementations call
FileReadV() and FileWriteV().  A range of disk blocks beginning at
'blocknum' and extending for 'nblocks' can be scattered to or gathered
from multiple buffers with a single system call.  The traditional
smgrread() and smgrwrite() functions are implemented in terms of the new
functions.

Later commits will introduce calls with nblocks > 1, but the following
behavioral changes can be seen already:

* After a short transfer we'll now retry until we eventually read 0
  bytes (= EOF) or get ENOSPC, EDQUOT, EFBIG etc, where previously we
  would infer the reason.  Retrying is consistent with xlog.c's
  treatment of large WAL writes, and arguably also xlog.c and fd.c's
  treatment of EINTR.  Arbitrary short returns for larger transfers have
  been observed on several OSes, and might in theory also happen for
  transient reasons with our own pg_p*v() fallback code.

* After unexpected EOF or -1, the error thrown now talks about
  a range even for the single block case, eg "blocks 42..42".

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/CA+hUKGJkOiOCa+mag4BF+zHo7qo=o9CFheB8=g6uT5TUm2gkvA@mail.gmail.com

2 files changed, 26 insertions, 8 deletions

diff --git a/src/include/storage/md.h b/src/include/storage/md.h
index 091784c171e..3dc651ec174 100644
--- a/src/include/storage/md.h
+++ b/src/include/storage/md.h
@@ -32,10 +32,11 @@ extern void mdzeroextend(SMgrRelation reln, ForkNumber forknum,
 						 BlockNumber blocknum, int nblocks, bool skipFsync);
 extern bool mdprefetch(SMgrRelation reln, ForkNumber forknum,
 					   BlockNumber blocknum, int nblocks);
-extern void mdread(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
-				   void *buffer);
-extern void mdwrite(SMgrRelation reln, ForkNumber forknum,
-					BlockNumber blocknum, const void *buffer, bool skipFsync);
+extern void mdreadv(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
+					void **buffers, BlockNumber nblocks);
+extern void mdwritev(SMgrRelation reln, ForkNumber forknum,
+					 BlockNumber blocknum,
+					 const void **buffers, BlockNumber nblocks, bool skipFsync);
 extern void mdwriteback(SMgrRelation reln, ForkNumber forknum,
 						BlockNumber blocknum, BlockNumber nblocks);
 extern BlockNumber mdnblocks(SMgrRelation reln, ForkNumber forknum);
diff --git a/src/include/storage/smgr.h b/src/include/storage/smgr.h
index da7e7c5a2e6..6e62dc053c7 100644
--- a/src/include/storage/smgr.h
+++ b/src/include/storage/smgr.h
@@ -96,10 +96,13 @@ extern void smgrzeroextend(SMgrRelation reln, ForkNumber forknum,
 						   BlockNumber blocknum, int nblocks, bool skipFsync);
 extern bool smgrprefetch(SMgrRelation reln, ForkNumber forknum,
 						 BlockNumber blocknum, int nblocks);
-extern void smgrread(SMgrRelation reln, ForkNumber forknum,
-					 BlockNumber blocknum, void *buffer);
-extern void smgrwrite(SMgrRelation reln, ForkNumber forknum,
-					  BlockNumber blocknum, const void *buffer, bool skipFsync);
+extern void smgrreadv(SMgrRelation reln, ForkNumber forknum,
+					  BlockNumber blocknum,
+					  void **buffer, BlockNumber nblocks);
+extern void smgrwritev(SMgrRelation reln, ForkNumber forknum,
+					   BlockNumber blocknum,
+					   const void **buffer, BlockNumber nblocks,
+					   bool skipFsync);
 extern void smgrwriteback(SMgrRelation reln, ForkNumber forknum,
 						  BlockNumber blocknum, BlockNumber nblocks);
 extern BlockNumber smgrnblocks(SMgrRelation reln, ForkNumber forknum);
@@ -110,4 +113,18 @@ extern void smgrimmedsync(SMgrRelation reln, ForkNumber forknum);
 extern void AtEOXact_SMgr(void);
 extern bool ProcessBarrierSmgrRelease(void);
 
+static inline void
+smgrread(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
+		 void *buffer)
+{
+	smgrreadv(reln, forknum, blocknum, &buffer, 1);
+}
+
+static inline void
+smgrwrite(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
+		  const void *buffer, bool skipFsync)
+{
+	smgrwritev(reln, forknum, blocknum, &buffer, 1, skipFsync);
+}
+
 #endif							/* SMGR_H */