Config option to turn AOF rewrite incremental fsync on/off.

redis.conf

@@ -673,6 +673,12 @@ client-output-buffer-limit pubsub 32mb 8mb 60
 # 100 only in environments where very low latency is required.
 hz 10
 
+# When a child rewrites the AOF file, if the following option is enabled
+# the file will be fsync-ed every 32 MB of data generated. This is useful
+# in order to commit the file to the disk more incrementally and avoid
+# big latency spikes.
+aof-rewrite-incremental-fsync yes
+
 ################################## INCLUDES ###################################
 
 # Include one or more other config files here.  This is useful if you

src/aof.c

@@ -855,7 +855,8 @@ int rewriteAppendOnlyFile(char *filename) {
     }
 
     rioInitWithFile(&aof,fp);
-    rioSetAutoSync(&aof,REDIS_AOF_AUTOSYNC_BYTES);
+    if (server.aof_rewrite_incremental_fsync)
+        rioSetAutoSync(&aof,REDIS_AOF_AUTOSYNC_BYTES);
     for (j = 0; j < server.dbnum; j++) {
         char selectcmd[] = "*2\r\n$6\r\nSELECT\r\n";
         redisDb *db = server.db+j;

src/config.c

@@ -330,6 +330,12 @@ void loadServerConfigFromString(char *config) {
                    argc == 2)
         {
             server.aof_rewrite_min_size = memtoll(argv[1],NULL);
+        } else if (!strcasecmp(argv[0],"aof-rewrite-incremental-fsync") &&
+                   argc == 2)
+        {
+            if ((server.aof_rewrite_incremental_fsync = yesnotoi(argv[1])) == -1) {
+                err = "argument must be 'yes' or 'no'"; goto loaderr;
+            }
         } else if (!strcasecmp(argv[0],"requirepass") && argc == 2) {
             if (strlen(argv[1]) > REDIS_AUTHPASS_MAX_LEN) {
                 err = "Password is longer than REDIS_AUTHPASS_MAX_LEN";
@@ -605,6 +611,11 @@ void configSetCommand(redisClient *c) {
     } else if (!strcasecmp(c->argv[2]->ptr,"auto-aof-rewrite-min-size")) {
         if (getLongLongFromObject(o,&ll) == REDIS_ERR || ll < 0) goto badfmt;
         server.aof_rewrite_min_size = ll;
+    } else if (!strcasecmp(c->argv[2]->ptr,"aof-rewrite-incremental-fsync")) {
+        int yn = yesnotoi(o->ptr);
+
+        if (yn == -1) goto badfmt;
+        server.aof_rewrite_incremental_fsync = yn;
     } else if (!strcasecmp(c->argv[2]->ptr,"save")) {
         int vlen, j;
         sds *v = sdssplitlen(o->ptr,sdslen(o->ptr)," ",1,&vlen);
@@ -899,6 +910,8 @@ void configGetCommand(redisClient *c) {
     config_get_bool_field("activerehashing", server.activerehashing);
     config_get_bool_field("repl-disable-tcp-nodelay",
             server.repl_disable_tcp_nodelay);
+    config_get_bool_field("aof-rewrite-incremental-fsync",
+            server.aof_rewrite_incremental_fsync);
 
     /* Everything we can't handle with macros follows. */
 

src/redis.c

@@ -1236,6 +1236,7 @@ void initServerConfig() {
     server.aof_fd = -1;
     server.aof_selected_db = -1; /* Make sure the first time will not match */
     server.aof_flush_postponed_start = 0;
+    server.aof_rewrite_incremental_fsync = 1;
     server.pidfile = zstrdup("/var/run/redis.pid");
     server.rdb_filename = zstrdup("dump.rdb");
     server.aof_filename = zstrdup("appendonly.aof");

src/redis.h

@@ -762,6 +762,7 @@ struct redisServer {
     time_t aof_rewrite_time_start;  /* Current AOF rewrite start time. */
     int aof_lastbgrewrite_status;   /* REDIS_OK or REDIS_ERR */
     unsigned long aof_delayed_fsync;  /* delayed AOF fsync() counter */
+    int aof_rewrite_incremental_fsync;/* fsync incrementally while rewriting? */
     /* RDB persistence */
     long long dirty;                /* Changes to DB from the last save */
     long long dirty_before_bgsave;  /* Used to restore dirty on failed BGSAVE */