document fjall db engine, remove flakey metadata_fsync implementation

doc/book/reference-manual/configuration.md

@@ -333,6 +333,7 @@ Since `v0.8.0`, Garage can use alternative storage backends as follows:
 | --------- | ----------------- | ------------- |
 | [LMDB](https://www.symas.com/lmdb) (since `v0.8.0`, default since `v0.9.0`) | `"lmdb"` | `<metadata_dir>/db.lmdb/` |
 | [Sqlite](https://sqlite.org) (since `v0.8.0`) | `"sqlite"` | `<metadata_dir>/db.sqlite` |
+| [Fjall](https://github.com/fjall-rs/fjall) (**experimental support** since `v1.3.0`) | `"fjall"` | `<metadata_dir>/db.fjall/` |
 | [Sled](https://sled.rs) (old default, removed since `v1.0`) | `"sled"` | `<metadata_dir>/db/` |
 
 Sled was supported until Garage v0.9.x, and was removed in Garage v1.0.
@@ -369,6 +370,14 @@ LMDB works very well, but is known to have the following limitations:
   so it is not the best choice for high-performance storage clusters,
   but it should work fine in many cases.
 
+- Fjall: a storage engine based on LSM trees, which theoretically allow for
+  higher write throughput than other storage engines that are based on B-trees.
+  Using Fjall could potentially improve Garage's performance significantly in
+  write-heavy workloads. **Support for Fjall is experimental at this point**,
+  we have added it to Garage for evaluation purposes only. **Do not use it for
+  production-critical workloads.**
+
+
 It is possible to convert Garage's metadata directory from one format to another
 using the `garage convert-db` command, which should be used as follows:
 
@@ -406,6 +415,7 @@ Here is how this option impacts the different database engines:
 |----------|------------------------------------|-------------------------------|
 | Sqlite   | `PRAGMA synchronous = OFF`         | `PRAGMA synchronous = NORMAL` |
 | LMDB     | `MDB_NOMETASYNC` + `MDB_NOSYNC`    | `MDB_NOMETASYNC`              |
+| Fjall    | default options                    | not supported                 |
 
 Note that the Sqlite database is always ran in `WAL` mode (`PRAGMA journal_mode = WAL`).
 

src/db/open.rs

@@ -124,8 +124,12 @@ pub fn open_db(path: &PathBuf, engine: Engine, opt: &OpenOpt) -> Result<Db> {
 		#[cfg(feature = "fjall")]
 		Engine::Fjall => {
 			info!("Opening Fjall database at: {}", path.display());
-			let fsync_ms = opt.fsync.then(|| 1000 as u16);
-			let mut config = fjall::Config::new(path).fsync_ms(fsync_ms);
+			if opt.fsync {
+				return Err(Error(
+					"metadata_fsync is not supported with the Fjall database engine".into(),
+				));
+			}
+			let mut config = fjall::Config::new(path);
 			if let Some(block_cache_size) = opt.fjall_block_cache_size {
 				config = config.cache_size(block_cache_size.try_into().unwrap());
 			}