[sql] Experiment to enable smart-vacuum on Android.

sql/connection.cc

Index: sql/connection.cc
diff --git a/sql/connection.cc b/sql/connection.cc
index 0511e11b5c5b22361bbdf24cc1230203749f272b..7500e6c9ebaeeeacc30a45ffcde7a441ad5351e6 100644
--- a/sql/connection.cc
+++ b/sql/connection.cc
@@ -14,6 +14,7 @@
 #include "base/bind.h"
 #include "base/debug/alias.h"
 #include "base/debug/dump_without_crashing.h"
+#include "base/feature_list.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/format_macros.h"
@@ -228,6 +229,15 @@ std::string AsUTF8ForSQL(const base::FilePath& path) {
 #endif
 }
 
+// http://crbug.com/698010
+//
+// The feature enables a SQLite patch which causes SQLite to leverage
+// SQLITE_FCNTL_CHUNK_SIZE when making auto-vacuum decisions.  This should lower
+// write load (histogram Sqlite.Vfs_Write in vfs_wrapper.cc).  Only Android
+// enables auto-vacuum under Chromium.
+const base::Feature kSqliteSmartAutoVacuumEnabled{
+    "SqliteSmartAutoVacuum", base::FEATURE_DISABLED_BY_DEFAULT};
+
 }  // namespace
 
 namespace sql {
@@ -1851,6 +1861,19 @@ bool Connection::OpenInternal(const std::string& file_name,
     if (db_size > 128 * 1024)
       chunk_size = 32 * 1024;
     sqlite3_file_control(db_, NULL, SQLITE_FCNTL_CHUNK_SIZE, &chunk_size);
+    if (base::FeatureList::IsEnabled(kSqliteSmartAutoVacuumEnabled)) {
+      int page_size = 0;
+      {
+        sql::Statement stmt(GetUniqueStatement("PRAGMA page_size"));
+        if (stmt.Step())
+          page_size = stmt.ColumnInt(0);
+        if (!page_size)
+          page_size = 1024;
+      }
+      std::string slack_sql = base::StringPrintf(
+          "PRAGMA auto_vacuum_slack_pages = %d", chunk_size / page_size);
+      ignore_result(Execute(slack_sql.c_str()));
+    }
   }
 
   // Enable memory-mapped access.  The explicit-disable case is because SQLite