[sql] Increase database chunk size for large databases.

sql/connection.cc

Index: sql/connection.cc
diff --git a/sql/connection.cc b/sql/connection.cc
index 79264916c0274c3cda466ccee94750b35e3f798d..3d584a65e11095a8e0c86becdcf16486fa3c306b 100644
--- a/sql/connection.cc
+++ b/sql/connection.cc
@@ -190,6 +190,16 @@ int GetSqlite3File(sqlite3* db, sqlite3_file** file) {
   return rc;
 }
 
+// Convenience to get the sqlite3_file* and the size for the "main" database.
+int GetSqlite3FileAndSize(sqlite3* db,
+                          sqlite3_file** file, sqlite3_int64* db_size) {
+  int rc = GetSqlite3File(db, file);
+  if (rc != SQLITE_OK)
+    return rc;
+
+  return (*file)->pMethods->xFileSize(*file, db_size);
+}
+
 // This should match UMA_HISTOGRAM_MEDIUM_TIMES().
 base::HistogramBase* GetMediumTimeHistogram(const std::string& name) {
   return base::Histogram::FactoryTimeGet(
@@ -515,12 +525,8 @@ void Connection::Preload() {
     return;
 
   sqlite3_file* file = NULL;
-  int rc = GetSqlite3File(db_, &file);
-  if (rc != SQLITE_OK)
-    return;
-
   sqlite3_int64 file_size = 0;
-  rc = file->pMethods->xFileSize(file, &file_size);
+  int rc = GetSqlite3FileAndSize(db_, &file, &file_size);
   if (rc != SQLITE_OK)
     return;
 
@@ -1656,6 +1662,23 @@ bool Connection::OpenInternal(const std::string& file_name,
     return false;
   }
 
+  // Set a reasonable chunk size for larger files.  This reduces churn from
+  // remapping memory on size changes.  It also reduces filesystem
+  // fragmentation.
+  // TODO(shess): It may make sense to have this be hinted by the client.
+  // Database sizes seem to be bimodal, some clients have consistently small
+  // databases (<20k) while other clients have a broad distribution of sizes
+  // (hundreds of kilobytes to many megabytes).
+  sqlite3_file* file = NULL;
+  sqlite3_int64 db_size = 0;
+  int rc = GetSqlite3FileAndSize(db_, &file, &db_size);
+  if (rc == SQLITE_OK && db_size > 16 * 1024) {
+    int chunk_size = 4 * 1024;
+    if (db_size > 128 * 1024)
+      chunk_size = 32 * 1024;
+    sqlite3_file_control(db_, NULL, SQLITE_FCNTL_CHUNK_SIZE, &chunk_size);
+  }
+
   // Enable memory-mapped access.  The explicit-disable case is because SQLite
   // can be built to default-enable mmap.  This value will be capped by
   // SQLITE_MAX_MMAP_SIZE, which could be different between 32-bit and 64-bit