Make fsync an option for meta and data

doc/book/reference-manual/configuration.md

@@ -10,6 +10,8 @@ Here is an example `garage.toml` configuration file that illustrates all of the
 ```toml
 metadata_dir = "/var/lib/garage/meta"
 data_dir = "/var/lib/garage/data"
+metadata_fsync = true
+data_fsync = false
 
 db_engine = "lmdb"
 
@@ -124,6 +126,49 @@ convert-db -a <input db engine> -i <input db path> \
 Make sure to specify the full database path as presented in the table above,
 and not just the path to the metadata directory.
 
+### `metadata_fsync`
+
+Whether to enable synchronous mode for the database engine or not.
+This is disabled (`false`) by default.
+
+This reduces the risk of metadata corruption in case of power failures,
+at the cost of a significant drop in write performance,
+as Garage will have to pause to sync data to disk much more often
+(several times for API calls such as PutObject).
+
+Using this option reduces the risk of simultaneous metadata corruption on several
+cluster nodes, which could lead to data loss.
+
+If multi-site replication is used, this option is most likely not necessary, as
+it is extremely unlikely that two nodes in different locations will have a 
+power failure at the exact same time.
+
+(Metadata corruption on a single node is not an issue, the corrupted data file
+can always be deleted and reconstructed from the other nodes in the cluster.)
+
+Here is how this option impacts the different database engines:
+
+| Database | `metadata_fsync = false` (default) | `metadata_fsync = true`       |
+|----------|------------------------------------|-------------------------------|
+| Sled     | default options                    | *unsupported*                 |
+| Sqlite   | `PRAGMA synchronous = OFF`         | `PRAGMA synchronous = NORMAL` |
+| LMDB     | `MDB_NOMETASYNC` + `MDB_NOSYNC`    | `MDB_NOMETASYNC`              |
+
+Note that the Sqlite database is always ran in `WAL` mode (`PRAGMA journal_mode = WAL`).
+
+### `data_fsync`
+
+Whether to `fsync` data blocks and their containing directory after they are
+saved to disk.
+This is disabled (`false`) by default.
+
+This might reduce the risk that a data block is lost in rare
+situations such as simultaneous node losing power,
+at the cost of a moderate drop in write performance.
+
+Similarly to `metatada_fsync`, this is likely not necessary
+if geographical replication is used.
+
 ### `block_size`
 
 Garage splits stored objects in consecutive chunks of size `block_size`

src/block/manager.rs

@@ -80,6 +80,7 @@ pub struct BlockManager {
 	/// Directory in which block are stored
 	pub data_dir: PathBuf,
 
+	data_fsync: bool,
 	compression_level: Option<i32>,
 
 	mutation_lock: [Mutex<BlockManagerLocked>; 256],
@@ -114,6 +115,7 @@ impl BlockManager {
 	pub fn new(
 		db: &db::Db,
 		data_dir: PathBuf,
+		data_fsync: bool,
 		compression_level: Option<i32>,
 		replication: TableShardedReplication,
 		system: Arc<System>,
@@ -141,6 +143,7 @@ impl BlockManager {
 		let block_manager = Arc::new(Self {
 			replication,
 			data_dir,
+			data_fsync,
 			compression_level,
 			mutation_lock: [(); 256].map(|_| Mutex::new(BlockManagerLocked())),
 			rc,
@@ -713,7 +716,11 @@ impl BlockManagerLocked {
 
 		let mut f = fs::File::create(&path_tmp).await?;
 		f.write_all(data).await?;
-		f.sync_all().await?;
+
+		if mgr.data_fsync {
+			f.sync_all().await?;
+		}
+
 		drop(f);
 
 		fs::rename(path_tmp, path).await?;
@@ -724,18 +731,20 @@ impl BlockManagerLocked {
 			fs::remove_file(to_delete).await?;
 		}
 
-		// We want to ensure that when this function returns, data is properly persisted
-		// to disk. The first step is the sync_all above that does an fsync on the data file.
-		// Now, we do an fsync on the containing directory, to ensure that the rename
-		// is persisted properly. See:
-		// http://thedjbway.b0llix.net/qmail/syncdir.html
-		let dir = fs::OpenOptions::new()
-			.read(true)
-			.mode(0)
-			.open(directory)
-			.await?;
-		dir.sync_all().await?;
-		drop(dir);
+		if mgr.data_fsync {
+			// We want to ensure that when this function returns, data is properly persisted
+			// to disk. The first step is the sync_all above that does an fsync on the data file.
+			// Now, we do an fsync on the containing directory, to ensure that the rename
+			// is persisted properly. See:
+			// http://thedjbway.b0llix.net/qmail/syncdir.html
+			let dir = fs::OpenOptions::new()
+				.read(true)
+				.mode(0)
+				.open(directory)
+				.await?;
+			dir.sync_all().await?;
+			drop(dir);
+		}
 
 		Ok(())
 	}

src/model/garage.rs

@@ -91,6 +91,11 @@ impl Garage {
 			// ---- Sled DB ----
 			#[cfg(feature = "sled")]
 			"sled" => {
+				if config.metadata_fsync {
+					return Err(Error::Message(format!(
+						"`metadata_fsync = true` is not supported with the Sled database engine"
+					)));
+				}
 				db_path.push("db");
 				info!("Opening Sled database at: {}", db_path.display());
 				let db = db::sled_adapter::sled::Config::default()
@@ -111,7 +116,11 @@ impl Garage {
 				let db = db::sqlite_adapter::rusqlite::Connection::open(db_path)
 					.and_then(|db| {
 						db.pragma_update(None, "journal_mode", &"WAL")?;
-						db.pragma_update(None, "synchronous", &"NORMAL")?;
+						if config.metadata_fsync {
+							db.pragma_update(None, "synchronous", &"NORMAL")?;
+						} else {
+							db.pragma_update(None, "synchronous", &"OFF")?;
+						}
 						Ok(db)
 					})
 					.ok_or_message("Unable to open sqlite DB")?;
@@ -139,6 +148,9 @@ impl Garage {
 				env_builder.map_size(map_size);
 				unsafe {
 					env_builder.flag(heed::flags::Flags::MdbNoMetaSync);
+					if !config.metadata_fsync {
+						env_builder.flag(heed::flags::Flags::MdbNoSync);
+					}
 				}
 				let db = match env_builder.open(&db_path) {
 					Err(heed::Error::Io(e)) if e.kind() == std::io::ErrorKind::OutOfMemory => {
@@ -208,6 +220,7 @@ impl Garage {
 		let block_manager = BlockManager::new(
 			&db,
 			config.data_dir.clone(),
+			config.data_fsync,
 			config.compression_level,
 			data_rep_param,
 			system.clone(),

src/util/config.rs

@@ -15,6 +15,13 @@ pub struct Config {
 	/// Path where to store data. Can be slower, but need higher volume
 	pub data_dir: PathBuf,
 
+	/// Whether to fsync after all metadata transactions (disabled by default)
+	#[serde(default)]
+	pub metadata_fsync: bool,
+	/// Whether to fsync after all data block writes (disabled by default)
+	#[serde(default)]
+	pub data_fsync: bool,
+
 	/// Size of data blocks to save to disk
 	#[serde(default = "default_block_size")]
 	pub block_size: usize,