ovsqlite: add mmapsize configuration

doc/pod/ovsqlite.pod

@@ -67,6 +67,17 @@ is consulted only when creating a new database.  Enabling compression
 saves about S<55 %> of disk space on standard overview data.  The default
 value is false.
 
+=item I<mmapsize>
+
+The maximum number of bytes of the database file that SQLite will
+memory-map.  When set, SQLite reads data directly from the operating
+system's page cache via mmap instead of copying it into its own page
+cache, which can significantly improve read performance for large
+databases.  A good starting value is the size of the database file
+or the amount of available memory, whichever is smaller.  For example,
+a value of C<4294967296> allows up to S<4 GB> to be memory-mapped.
+The default value of 0 disables memory-mapped I/O.
+
 =item I<pagesize>
 
 The SQLite database page size in bytes.  Must be a power of 2, minimum 512,

samples/ovsqlite.conf

@@ -10,6 +10,13 @@
 # The default value is false.
 #compress:              false
 
+# The maximum number of bytes of the database file that SQLite will
+# memory-map.  When set, SQLite reads data via mmap instead of copying
+# into its own page cache, which can improve read performance for large
+# databases.  A good value is the database file size or available memory,
+# whichever is smaller.  Set to 0 (the default) to disable.
+#mmapsize:              0
+
 # The SQLite database page size in bytes.
 # Must be a power of 2, minimum 512, maximum 65536.
 # Appropriate values include the virtual memory page size and the

storage/ovsqlite/ovsqlite-server.c

@@ -119,6 +119,7 @@ static sql_main_t sql_main;
 static bool use_compression;
 static unsigned long pagesize;
 static unsigned long cachesize;
+static unsigned long mmapsize;
 static struct timeval transaction_time_limit = {10, 0};
 static unsigned long transaction_row_limit = 10000;
 
@@ -386,6 +387,7 @@ load_config(void)
         config_param_boolean(top, "compress", &use_compression);
         config_param_unsigned_number(top, "pagesize", &pagesize);
         config_param_unsigned_number(top, "cachesize", &cachesize);
+        config_param_unsigned_number(top, "mmapsize", &mmapsize);
         if (config_param_real(top, "transtimelimit", &timelimit)) {
             transaction_time_limit.tv_sec = (long) timelimit;
             transaction_time_limit.tv_usec =
@@ -656,6 +658,15 @@ open_db(void)
             sqlite3_free(errmsg);
         }
     }
+    if (mmapsize) {
+        snprintf(sqltext, sizeof sqltext, "pragma mmap_size = %lu;",
+                 mmapsize);
+        status = sqlite3_exec(connection, sqltext, 0, NULL, &errmsg);
+        if (status != SQLITE_OK) {
+            warn("cannot set mmap size: %s", errmsg);
+            sqlite3_free(errmsg);
+        }
+    }
 }
 
 static void