From 2c741eafd08ee659f80c2a8617221bac2fcec111 Mon Sep 17 00:00:00 2001
From: Wouter Verhelst <w@uter.be>
Date: Fri, 17 Jul 2009 10:58:03 +0200
Subject: [PATCH] Implement sync option

This allows a user to enable fsync() calls after a write to a backend
file.
---
 nbd-server.5.in   |   16 +++++++++++++++-
 nbd-server.5.sgml |   17 +++++++++++++++++
 nbd-server.c      |   14 +++++++++++---
 3 files changed, 43 insertions(+), 4 deletions(-)

diff --git a/nbd-server.5.in b/nbd-server.5.in
index ddcede2..5d461ad 100644
--- a/nbd-server.5.in
+++ b/nbd-server.5.in
@@ -3,7 +3,7 @@
 .\" <http://shell.ipoline.com/~elmert/comp/docbook2X/> 
 .\" Please send any bug reports, improvements, comments, patches, 
 .\" etc. to Steve Cheng <steve@ggi-project.org>.
-.TH "NBD-SERVER" "5" "03 January 2009" "" ""
+.TH "NBD-SERVER" "5" "17 July 2009" "" ""
 
 .SH NAME
 /etc/nbd-server/config \- configuration file for nbd-server
@@ -212,6 +212,20 @@ is found in a configuration file and
 then \fBnbd-server\fR will exit with an error
 message.
 .TP
+\fBsync\fR
+Optional; boolean.
+
+When this option is enabled,
+\fBnbd-server\fR will call an fsync() after every
+write to the backend storage. Calling fsync() increases
+reliability in case of an unclean shutdown of nbd-server; but,
+depending on the file system used on the nbd-server side, may
+degrade performance. The use of this option isn't always
+necessary; e.g., on ext3 filesystems, it is recommended that
+it is \fBnot\fR enabled, since it seriously
+reduces performance on ext3 filesystems while not
+importantly impacting performance.
+.TP
 \fBsparse_cow\fR
 Optional; boolean.
 
diff --git a/nbd-server.5.sgml b/nbd-server.5.sgml
index 47c65ee..fa9b321 100644
--- a/nbd-server.5.sgml
+++ b/nbd-server.5.sgml
@@ -329,6 +329,23 @@ manpage.1: manpage.sgml
 	</listitem>
       </varlistentry>
       <varlistentry>
+        <term><option>sync</option></term>
+	<listitem>
+	  <para>Optional; boolean.</para>
+	  <para>When this option is enabled,
+	    <command>nbd-server</command> will call an fsync() after every
+	    write to the backend storage. Calling fsync() increases
+	    reliability in case of an unclean shutdown of nbd-server; but,
+	    depending on the file system used on the nbd-server side, may
+	    degrade performance. The use of this option isn't always
+	    necessary; e.g., on ext3 filesystems, it is recommended that
+	    it is <emphasis>not</emphasis> enabled, since it seriously
+	    reduces performance on ext3 filesystems while not
+	    importantly impacting performance.
+	  </para>
+	</listitem>
+      </varlistentry>
+      <varlistentry>
 	<term><option>sparse_cow</option></term>
 	<listitem>
 	  <para>Optional; boolean.</para>
diff --git a/nbd-server.c b/nbd-server.c
index 4426345..58c5cc5 100644
--- a/nbd-server.c
+++ b/nbd-server.c
@@ -149,6 +149,7 @@ gchar* rungroup=NULL;
 #define F_AUTOREADONLY 8  /**< flag to tell us a file is set to autoreadonly */
 #define F_SPARSE 16	  /**< flag to tell us copyronwrite should use a sparse file */
 #define F_SDP 32	  /**< flag to tell us the export should be done using the Socket Direct Protocol for RDMA */
+#define F_SYNC 64	  /**< Whether to fsync() after a write */
 GHashTable *children;
 char pidfname[256]; /**< name of our PID file */
 char pidftemplate[256]; /**< template to be used for the filename of the PID file */
@@ -550,6 +551,7 @@ GArray* parse_cfile(gchar* f, GError** e) {
 		{ "copyonwrite", FALSE,	PARAM_BOOL,	NULL, F_COPYONWRITE },
 		{ "sparse_cow",	FALSE,	PARAM_BOOL,	NULL, F_SPARSE },
 		{ "sdp",	FALSE,	PARAM_BOOL,	NULL, F_SDP },
+		{ "sync",	FALSE,  PARAM_BOOL,	NULL, F_SYNC },
 		{ "listenaddr", FALSE,  PARAM_STRING,   NULL, 0 },
 	};
 	const int lp_size=sizeof(lp)/sizeof(PARAM);
@@ -596,8 +598,9 @@ GArray* parse_cfile(gchar* f, GError** e) {
 		lp[5].target=&(s.prerun);
 		lp[6].target=&(s.postrun);
 		lp[7].target=lp[8].target=lp[9].target=
-				lp[10].target=lp[11].target=&(s.flags);
-		lp[12].target=&(s.listenaddr);
+				lp[10].target=lp[11].target=
+				lp[12].target=&(s.flags);
+		lp[13].target=&(s.listenaddr);
 
 		/* After the [generic] group, start parsing exports */
 		if(i==1) {
@@ -883,6 +886,7 @@ ssize_t rawexpwrite(off_t a, char *buf, size_t len, CLIENT *client) {
 	int fhandle;
 	off_t foffset;
 	size_t maxbytes;
+	ssize_t retval;
 
 	if(get_filepos(client->export, a, &fhandle, &foffset, &maxbytes))
 		return -1;
@@ -892,7 +896,11 @@ ssize_t rawexpwrite(off_t a, char *buf, size_t len, CLIENT *client) {
 	DEBUG4("(WRITE to fd %d offset %llu len %u), ", fhandle, foffset, len);
 
 	myseek(fhandle, foffset);
-	return write(fhandle, buf, len);
+	retval = write(fhandle, buf, len);
+	if(client->server->flags & F_SYNC) {
+		fsync(fhandle);
+	}
+	return retval;
 }
 
 /**
-- 
1.7.10.4