* Use SQLite 3.7.0's write-ahead logging (WAL mode). This is a lot

  faster than the old mode when fsyncs are enabled, because it only
  performs an fsync() when doing a checkpoint, rather than at every
  commit.  Some timings for doing a "nix-instantiate /etc/nixos/nixos
  -A system" after modifying the stdenv setup script:

    42.5s - SQLite 3.6.23 with truncate mode and fsync
     3.4s - SQLite 3.6.23 with truncate mode and no fsync
    32.1s - SQLite 3.7.0 with truncate mode and fsync
    16.8s - SQLite 3.7.0 with WAL mode and fsync, auto-checkpoint
            every 1000 pages
     8.3s - SQLite 3.7.0 with WAL mode and fsync, auto-checkpoint
            every 8192 pages
     1.7s - SQLite 3.7.0 with WAL mode and no fsync

  The default is now to use WAL mode with fsyncs.  Because WAL doesn't
  work on remote filesystems such as NFS (as it uses shared memory),
  truncate mode can be re-enabled by setting the "use-sqlite-wal"
  option to false.

1 files changed, 12 insertions, 2 deletions

diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 6f3d9efa8..bddcbad12 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -298,10 +298,20 @@ void LocalStore::openDB(bool create)
     if (sqlite3_exec(db, ("pragma synchronous = " + syncMode + ";").c_str(), 0, 0, 0) != SQLITE_OK)
         throw SQLiteError(db, "setting synchronous mode");
 
-    /* Use `truncate' journal mode, which should be a bit faster. */
-    if (sqlite3_exec(db, "pragma main.journal_mode = truncate;", 0, 0, 0) != SQLITE_OK)
+    /* Set the SQLite journal mode.  The default is write-ahead
+       logging since it's the fastest and supports more concurrency.
+       The downside is that it doesn't work over NFS, so allow
+       truncate mode alternatively. */
+    string mode = queryBoolSetting("use-sqlite-wal", true) ? "wal" : "truncate";
+    if (sqlite3_exec(db, ("pragma main.journal_mode = " + mode + ";").c_str(), 0, 0, 0) != SQLITE_OK)
         throw SQLiteError(db, "setting journal mode");
 
+    /* Increase the auto-checkpoint interval to 8192 pages.  This
+       seems enough to ensure that instantiating the NixOS system
+       derivation is done in a single fsync(). */
+    if (sqlite3_exec(db, "pragma wal_autocheckpoint = 8192;", 0, 0, 0) != SQLITE_OK)
+        throw SQLiteError(db, "setting autocheckpoint interval");
+    
     /* Initialise the database schema, if necessary. */
     if (create) {
 #include "schema.sql.hh"