Merge pull request 'Doc: be slightly more critical of LMDB' (#773) from doc-updates into main

Reviewed-on: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/773

4 files changed, 55 insertions, 33 deletions

diff --git a/doc/book/cookbook/from-source.md b/doc/book/cookbook/from-source.md
index bacf93ab..ade47e96 100644
--- a/doc/book/cookbook/from-source.md
+++ b/doc/book/cookbook/from-source.md
@@ -91,5 +91,5 @@ The following feature flags are available in v0.8.0:
 | `metrics` | *by default* | Enable collection of metrics in Prometheus format on the admin API |
 | `telemetry-otlp` | optional | Enable collection of execution traces using OpenTelemetry |
 | `sled` | *by default* | Enable using Sled to store Garage's metadata |
-| `lmdb` | optional | Enable using LMDB to store Garage's metadata |
-| `sqlite` | optional | Enable using Sqlite3 to store Garage's metadata |
+| `lmdb` | *by default* | Enable using LMDB to store Garage's metadata |
+| `sqlite` | *by default* | Enable using Sqlite3 to store Garage's metadata |
diff --git a/doc/book/cookbook/real-world.md b/doc/book/cookbook/real-world.md
index c15ea384..15a58b9b 100644
--- a/doc/book/cookbook/real-world.md
+++ b/doc/book/cookbook/real-world.md
@@ -27,7 +27,7 @@ To run a real-world deployment, make sure the following conditions are met:
   [Yggdrasil](https://yggdrasil-network.github.io/) are approaches to consider
   in addition to building out your own VPN tunneling.
 
-- This guide will assume you are using Docker containers to deploy Garage on each node. 
+- This guide will assume you are using Docker containers to deploy Garage on each node.
   Garage can also be run independently, for instance as a [Systemd service](@/documentation/cookbook/systemd.md).
   You can also use an orchestrator such as Nomad or Kubernetes to automatically manage
   Docker containers on a fleet of nodes.
@@ -53,9 +53,9 @@ to store 2 TB of data in total.
 
 ### Best practices
 
-- If you have fast dedicated networking between all your nodes, and are planing to store
-  very large files, bump the `block_size` configuration parameter to 10 MB
-  (`block_size = 10485760`).
+- If you have reasonably fast networking between all your nodes, and are planing to store
+  mostly large files, bump the `block_size` configuration parameter to 10 MB
+  (`block_size = "10M"`).
 
 - Garage stores its files in two locations: it uses a metadata directory to store frequently-accessed
   small metadata items, and a data directory to store data blocks of uploaded objects.
@@ -68,20 +68,29 @@ to store 2 TB of data in total.
   EXT4 is not recommended as it has more strict limitations on the number of inodes,
   which might cause issues with Garage when large numbers of objects are stored.
 
-- If you only have an HDD and no SSD, it's fine to put your metadata alongside the data
-  on the same drive. Having lots of RAM for your kernel to cache the metadata will
-  help a lot with performance.  Make sure to use the LMDB database engine,
-  instead of Sled, which suffers from quite bad performance degradation on HDDs.
-  Sled is still the default for legacy reasons, but is not recommended anymore.
-
-- For the metadata storage, Garage does not do checksumming and integrity
-  verification on its own. If you are afraid of bitrot/data corruption,
-  put your metadata directory on a ZFS or BTRFS partition. Otherwise, just use regular
-  EXT4 or XFS.
-
 - Servers with multiple HDDs are supported natively by Garage without resorting
   to RAID, see [our dedicated documentation page](@/documentation/operations/multi-hdd.md).
 
+- For the metadata storage, Garage does not do checksumming and integrity
+  verification on its own. Users have reported that when using the LMDB
+  database engine (the default), database files have a tendency of becoming
+  corrupted after an unclean shutdown (e.g. a power outage), so you should use
+  a robust filesystem such as BTRFS or ZFS for the metadata partition, and take
+  regular snapshots so that you can restore to a recent known-good state in
+  case of an incident.  If you cannot do so, you might want to switch to Sqlite
+  which is more robust.
+
+- LMDB is the fastest and most tested database engine, but it has the following
+  weaknesses: 1/ data files are not architecture-independent, you cannot simply
+  move a Garage metadata directory between nodes running different architectures,
+  and 2/ LMDB is not suited for 32-bit platforms. Sqlite is a viable alternative
+  if any of these are of concern.
+
+- If you only have an HDD and no SSD, it's fine to put your metadata alongside
+  the data on the same drive, but then consider your filesystem choice wisely
+  (see above).  Having lots of RAM for your kernel to cache the metadata will
+  help a lot with performance.
+
 ## Get a Docker image
 
 Our docker image is currently named `dxflrs/garage` and is stored on the [Docker Hub](https://hub.docker.com/r/dxflrs/garage/tags?page=1&ordering=last_updated).
@@ -187,7 +196,7 @@ upgrades.  With the containerized setup proposed here, the upgrade process
 will require stopping and removing the existing container, and re-creating it
 with the upgraded version.
 
-## Controling the daemon
+## Controlling the daemon
 
 The `garage` binary has two purposes:
   - it acts as a daemon when launched with `garage server`
@@ -245,7 +254,7 @@ You can then instruct nodes to connect to one another as follows:
 Venus$ garage node connect 563e1ac825ee3323aa441e72c26d1030d6d4414aeb3dd25287c531e7fc2bc95d@[fc00:1::1]:3901
 ```
 
-You don't nead to instruct all node to connect to all other nodes:
+You don't need to instruct all node to connect to all other nodes:
 nodes will discover one another transitively.
 
 Now if your run `garage status` on any node, you should have an output that looks as follows:
@@ -328,8 +337,8 @@ Given the information above, we will configure our cluster as follow:
 ```bash
 garage layout assign 563e -z par1 -c 1T -t mercury
 garage layout assign 86f0 -z par1 -c 2T -t venus
-garage layout assign 6814 -z lon1 -c 2T -t earth 
-garage layout assign 212f -z bru1 -c 1.5T -t mars 
+garage layout assign 6814 -z lon1 -c 2T -t earth
+garage layout assign 212f -z bru1 -c 1.5T -t mars
 ```
 
 At this point, the changes in the cluster layout have not yet been applied.
diff --git a/doc/book/quick-start/_index.md b/doc/book/quick-start/_index.md
index f359843d..870bf9e9 100644
--- a/doc/book/quick-start/_index.md
+++ b/doc/book/quick-start/_index.md
@@ -57,7 +57,7 @@ to generate unique and private secrets for security reasons:
 cat > garage.toml <<EOF
 metadata_dir = "/tmp/meta"
 data_dir = "/tmp/data"
-db_engine = "lmdb"
+db_engine = "sqlite"
 
 replication_mode = "none"
 
diff --git a/doc/book/reference-manual/configuration.md b/doc/book/reference-manual/configuration.md
index af7690f4..81af1de0 100644
--- a/doc/book/reference-manual/configuration.md
+++ b/doc/book/reference-manual/configuration.md
@@ -264,18 +264,31 @@ Performance characteristics of the different DB engines are as follows:
 - 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.
 
-- 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.
+- LMDB: the recommended database engine for high-performance distributed
+  clusters, much more space-efficient and significantly faster. LMDB works very
+  well, but is known to have the following limitations:
+
+  - 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.
+
+  - 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.
+
+  - Several users have reported corrupted LMDB database files after an unclean
+    shutdown (e.g. a power outage). This situation can generally be recovered
+    from if your cluster is geo-replicated (by rebuilding your metadata db from
+    other nodes), or if you have saved regular snapshots at the filesystem
+    level.
 
 - 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.
+  metadata, which does not have the issues listed above for LMDB.
+  On versions 0.8.x and earlier, Sqlite should be avoided due to abysmal
+  performance, which was fixed with the addition of `metadata_fsync`.
+  Sqlite is still probably slower than LMDB due to the way we use it,
+  so it is not the best choice for high-performance storage clusters,
+  but it should work fine in many cases.
 
 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:
@@ -302,7 +315,7 @@ Using this option reduces the risk of simultaneous metadata corruption on severa
 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 
+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