pg_test_fsync: add C comment about direct I/O and write size failure

Report from Marti Raudsepp

diff --git a/contrib/pg_test_fsync/pg_test_fsync.c b/contrib/pg_test_fsync/pg_test_fsync.c
index 669c4190d38c342afaaa0e4480dfc374612d6063..682821a8d787392fc4fa9ffa99e00d2cb8b77067 100644 (file)
--- a/contrib/pg_test_fsync/pg_test_fsync.c
+++ b/contrib/pg_test_fsync/pg_test_fsync.c
@@ -369,6 +369,12 @@ test_sync(int writes_per_op)
        {
            for (writes = 0; writes < writes_per_op; writes++)
                if (write(tmpfile, buf, XLOG_BLCKSZ) != XLOG_BLCKSZ)
+               /*
+                * This can generate write failures if the filesystem
+                * has a large block size, e.g. 4k, and there is no
+                * support for O_DIRECT writes smaller than the
+                * file system block size, e.g. XFS.
+                */
                    die("write failed");
            if (lseek(tmpfile, 0, SEEK_SET) == -1)
                die("seek failed");