From f9d77b6cd9e447af05650945d620bced0bda5c3c Mon Sep 17 00:00:00 2001 From: Alex Auvolat Date: Fri, 2 Apr 2021 19:29:51 +0200 Subject: Section on recovering from failures --- doc/book/src/cookbook/recovering.md | 98 +++++++++++++++++++++++++++++++++++++ 1 file changed, 98 insertions(+) create mode 100644 doc/book/src/cookbook/recovering.md (limited to 'doc/book/src/cookbook') diff --git a/doc/book/src/cookbook/recovering.md b/doc/book/src/cookbook/recovering.md new file mode 100644 index 00000000..4472f802 --- /dev/null +++ b/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 +``` + +(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 -c -d -t +``` + +Garage will then start synchronizing all required data on the new node. +This process can be monitored using the `garage stats -a` command. -- cgit v1.2.3