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Doc: fix db_engines section and improve config reference #674

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doc/book/reference-manual/configuration.md
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@ -107,44 +107,47 @@ on how to operate Garage in such a setup.
### `db_engine` (since `v0.8.0`)
By default, Garage uses the Sled embedded database library
to store its metadata on-disk. Since `v0.8.0`, Garage can use alternative storage backends as follows:
Since `v0.8.0`, Garage can use alternative storage backends as follows:
| DB engine | `db_engine` value | Database path |
| --------- | ----------------- | ------------- |
| [Sled](https://sled.rs) | `"sled"` | `<metadata_dir>/db/` |
| [LMDB](https://www.lmdb.tech) | `"lmdb"` | `<metadata_dir>/db.lmdb/` |
| [LMDB](https://www.lmdb.tech) (default since `v0.9.0`) | `"lmdb"` | `<metadata_dir>/db.lmdb/` |
| [Sled](https://sled.rs) (default up to `v0.8.0`) | `"sled"` | `<metadata_dir>/db/` |
| [Sqlite](https://sqlite.org) | `"sqlite"` | `<metadata_dir>/db.sqlite` |
Sled was the only database engine up to Garage v0.7.0. Performance issues and
API limitations of Sled prompted the addition of alternative engines in v0.8.0.
Since v0.9.0, LMDB is the default engine instead of Sled, and Sled is
deprecated. We plan to remove Sled in Garage v1.0.
Performance characteristics of the different DB engines are as follows:
- Sled: the default database engine, which tends to produce
large data files and also has performance issues, especially when the metadata folder
is on a traditional HDD and not on SSD.
- LMDB: the recommended alternative on 64-bit systems,
much more space-efficiant and slightly faster. Note that the data format of LMDB is not portable
between architectures, so for instance the Garage database of an x86-64
node cannot be moved to an ARM64 node. Also note that, while LMDB can technically be used on 32-bit systems,
this will limit your node to very small database sizes due to how LMDB works; it is therefore not recommended.
- Sqlite: Garage supports Sqlite as a storage backend for metadata,
however it may have issues and is also very slow in its current implementation,
so it is not recommended to be used for now.
- Sled: tends to produce large data files and also has performance issues,
especially when the metadata folder is on a traditional HDD and not on SSD.
It is possible to convert Garage's metadata directory from one format to another with a small utility named `convert_db`,
which can be downloaded at the following locations:
[for amd64](https://garagehq.deuxfleurs.fr/_releases/convert_db/amd64/convert_db),
[for i386](https://garagehq.deuxfleurs.fr/_releases/convert_db/i386/convert_db),
[for arm64](https://garagehq.deuxfleurs.fr/_releases/convert_db/arm64/convert_db),
[for arm](https://garagehq.deuxfleurs.fr/_releases/convert_db/arm/convert_db).
The `convert_db` utility is used as folows:
- LMDB: the recommended database engine on 64-bit systems, much more
space-efficient and slightly faster. Note that the data format of LMDB is not
portable between architectures, so for instance the Garage database of an
x86-64 node cannot be moved to an ARM64 node. Also note that, while LMDB can
technically be used on 32-bit systems, this will limit your node to very
small database sizes due to how LMDB works; it is therefore not recommended.
- Sqlite: Garage supports Sqlite as an alternative storage backend for
metadata, and although it has not been tested as much, it is expected to work
satisfactorily. Since Garage v0.9.0, performance issues have largely been
fixed by allowing for a no-fsync mode (see `metadata_fsync`). Sqlite does not
have the database size limitation of LMDB on 32-bit systems.
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:
```
convert-db -a <input db engine> -i <input db path> \
garage convert-db -a <input db engine> -i <input db path> \
-b <output db engine> -o <output 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.
Make sure to specify the full database path as presented in the table above
(third colummn), and not just the path to the metadata directory.
### `metadata_fsync`