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