initdb: Further polishing of --wal-segsize option

Extend documentation.  Improve option parsing in case no argument was
specified.

doc/src/sgml/ref/initdb.sgml

@@ -316,16 +316,22 @@ PostgreSQL documentation
      </varlistentry>
 
      <varlistentry>
-       <term><option>--wal-segsize=<replaceable>SEGSIZE</replaceable></option></term>
+       <term><option>--wal-segsize=<replaceable>size</replaceable></option></term>
        <listitem>
         <para>
-         Set the <firstterm>WAL segment size</firstterm>, in megabytes.  This is
-         the size of each individual file in the WAL log.  It may be useful
-         to adjust this size to control the granularity of WAL log shipping.
-         This option can only be set during initialization, and cannot be
-         changed later.
-         The default size is 16 megabytes.
-         The value must be a power of 2 between 1 and 1024 (megabytes).
+         Set the <firstterm>WAL segment size</firstterm>, in megabytes.  This
+         is the size of each individual file in the WAL log.  The default size
+         is 16 megabytes.  The value must be a power of 2 between 1 and 1024
+         (megabytes).  This option can only be set during initialization, and
+         cannot be changed later.
+        </para>
+
+        <para>
+         It may be useful to adjust this size to control the granularity of
+         WAL log shipping or archiving.  Also, in databases with a high volume
+         of WAL, the sheer number of WAL files per directory can become a
+         performance and management problem.  Increasing the WAL file size
+         will reduce the number of WAL files.
         </para>
        </listitem>
       </varlistentry>

src/bin/initdb/initdb.c

@@ -3224,7 +3224,7 @@ main(int argc, char *argv[])
 		wal_segment_size_mb = strtol(str_wal_segment_size_mb, &endptr, 10);
 
 		/* verify that wal segment size is valid */
-		if (*endptr != '\0')
+		if (endptr == str_wal_segment_size_mb || *endptr != '\0')
 		{
 			fprintf(stderr,
 					_("%s: argument of --wal-segsize must be a number\n"),
@@ -3234,7 +3234,7 @@ main(int argc, char *argv[])
 		if (!IsValidWalSegSize(wal_segment_size_mb * 1024 * 1024))
 		{
 			fprintf(stderr,
-					_("%s: argument of --wal-segsize must be a power of two between 1 and 1024\n"),
+					_("%s: argument of --wal-segsize must be a power of 2 between 1 and 1024\n"),
 					progname);
 			exit(1);
 		}