Implement sync option

This allows a user to enable fsync() calls after a write to a backend
file.

diff --git a/nbd-server.5.in b/nbd-server.5.in
index ddcede2..5d461ad 100644 (file)
--- 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 (file)
--- 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 (file)
--- 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;
 }
 
 /**