Section on recovering from failures

doc/book/src/SUMMARY.md

@@ -14,6 +14,7 @@
   - [Host a website](./cookbook/website.md)
   - [Integrate as a media backend]()
   - [Operate a cluster]()
+  - [Recovering from failures](./cookbook/recovering.md)
 
 - [Reference Manual](./reference_manual/index.md)
   - [Garage CLI]()

doc/book/src/cookbook/recovering.md

@@ -0,0 +1,98 @@
+# Recovering from failures
+
+Garage is meant to work on old, second-hand hardware.
+In particular, this makes it likely that some of your drives will fail, and some manual intervention will be needed.
+Fear not! For Garage is fully equipped to handle drive failures, in most common cases.
+
+## A note on availability of Garage
+
+With nodes dispersed in 3 datacenters or more, here are the guarantees Garage provides with the default replication strategy (3 copies of all data, which is the recommended value):
+
+- The cluster remains fully functionnal as long as the machines that fail are in only one datacenter. This includes a whole datacenter going down due to power/Internet outage.
+- No data is lost as long as the machines that fail are in at most two datacenters.
+
+Of course this only works if your Garage nodes are correctly configured to be aware of the datacenter in which they are located.
+Make sure this is the case using `garage status` to check on the state of your cluster's configuration.
+
+
+## First option: removing a node
+
+If you don't have spare parts (HDD, SDD) to replace the failed component, and if there are enough remaining nodes in your cluster
+(at least 3), you can simply remove the failed node from Garage's configuration.
+Note that if you **do** intend to replace the failed parts by new ones, using this method followed by adding back the node is **not recommended** (although it should work),
+and you should instead use one of the methods detailed in the next sections.
+
+Removing a node is done with the following command:
+
+```
+garage node remove --yes <node_id>
+```
+
+(you can get the `node_id` of the failed node by running `garage status`)
+
+This will repartition the data and ensure that 3 copies of everything are present on the nodes that remain available.
+
+
+
+## Replacement scenario 1: only data is lost, metadata is fine
+
+The recommended deployment for Garage uses an SSD to store metadata, and an HDD to store blocks of data.
+In the case where only a single HDD crashes, the blocks of data are lost but the metadata is still fine.
+
+This is very easy to recover by setting up a new HDD to replace the failed one.
+The node does not need to be fully replaced and the configuration doesn't need to change.
+We just need to tell Garage to get back all the data blocks and store them on the new HDD.
+
+First, set up a new HDD to store Garage's data directory on the failed node, and restart Garage using
+the existing configuration.  Then, run:
+
+```
+garage repair -a --yes blocks
+```
+
+This will re-synchronize blocks of data that are missing to the new HDD, reading them from copies located on other nodes.
+
+You can check on the advancement of this process by doing the following command: 
+
+```
+garage stats -a
+```
+
+Look out for the following output:
+
+```
+Block manager stats:
+  resync queue length: 26541
+```
+
+This indicates that one of the Garage node is in the process of retrieving missing data from other nodes.
+This number decreases to zero when the node is fully synchronized.
+
+
+## Replacement scenario 2: metadata (and possibly data) is lost
+
+This scenario covers the case where a full node fails, i.e. both the metadata directory and
+the data directory are lost, as well as the case where only the metadata directory is lost.
+
+To replace the lost node, we will start from an empty metadata directory, which means
+Garage will generate a new node ID for the replacement node.
+We will thus need to remove the previous node ID from Garage's configuration and replace it by the ID of the new node.
+
+If your data directory is stored on a separate drive and is still fine, you can keep it, but it is not necessary to do so.
+In all cases, the data will be rebalanced and the replacement node will not store the same pieces of data
+as were originally stored on the one that failed. So if you keep the data files, the rebalancing
+might be faster but most of the pieces will be deleted anyway from the disk and replaced by other ones.
+
+First, set up a new drive to store the metadata directory for the replacement node (a SSD is recommended),
+and for the data directory if necessary. You can then start Garage on the new node.
+The restarted node should generate a new node ID, and it should be shown as `NOT CONFIGURED` in `garage status`.
+The ID of the lost node should be shown in `garage status` in the section for disconnected/unavailable nodes.
+
+Then, replace the broken node by the new one, using:
+
+```
+garage node add --replace <old_node_id> -c <capacity> -d <datacenter> -t <node_tag> <new_node_id>
+```
+
+Garage will then start synchronizing all required data on the new node.
+This process can be monitored using the `garage stats -a` command.