# Restarting a Large Cluster that Has Dropped Multiple TiKV Nodes

## Summary

-------

On occasions, such as when an AWS availability zone - or even a whole region - goes down, or when the RocksDB component encounters a memory region that is an inconsistent state, this may cause the entire cluster to shut down with one or more errors posted.

## Diagnosis

---------

When an availability zone goes down, all the cluster nodes from that zone will also go down and will have to be failed over to another availability zone. If, on the other hand, the issue is with data consistency at the storage, then the TiKV instance will go down with an error like the following:

```
[_timestamp_] [FATAL] [server.rs:708] ["failed to start node: Engine(Other("[components/raftstore/src/store/fsm/store.rs:932\]: Engine Engine("Corruption: block checksum mismatch: expected 417475072, got 2324967102  in /mnt/data/ti/data/tikv-20160/raft/1537737.sst offset 6145532 size 17374")"))"]
```

In that case, the host may be available, but it will refuse to restart with the same error because of the data corruption.  In addition, upon restarting the TiDB service, you may see a failure with complaints about `low-space-ratio`. This means that the service does not think there is sufficient empty space in a working volume to accommodate future needs.

## Solution

--------

When you are faced with a situation where one or more TiKV nodes have dropped off, be sure to replace the failing nodes before attempting to restart the cluster. This will avoid repeated failed restart attempts due to under-provisioned TiKV nodes.

Sometimes, in situations where your hosts have a lot of disk storage, TiKV may fail to start with a message about `low-space-ratio` in the logs. When this condition occurs, you can correct it by checking the `low-space-ratio` setting in the [PD configuration file](https://docs.pingcap.com/tidb/stable/pd-configuration-file#low-space-ratio).

The following is a sample command for obtaining the PD Configuration for a PD host named ‘pd1.pingcap.com’:

```{.copyable .custom-prompt .prompt-tiup}
> ctl:v5.2.1 pd config show all -u http://pd1.pingcap.com:2379 | jq '.schedule."low-space-ratio"'
```

```output
> 0.8
```

You can see that the `low-space-ratio` is set to the default of 0.8. This Means that, If your PD host has a 3TB partition, you may have as much as 600 MB of free space and still not meet your `low-space-ratio` requirement.

You can correct this by changing the ‘low-space-partition’ setting to ‘0.9’, which should allow the PD to start with a lot less free storage space (<= 300 MB, in this case) with the following command

```{.copyable .custom-prompt .prompt-tiup}
> ctl:v5.2.1 pd config set low-space-ratio 0.9 -u http://pd1.pingcap.com:2379
```

```output
> Starting component `ctl`: /home/ubuntu/.tiup/components/ctl/v5.2.1/ctl pd config set low-space-ratio 0.9 -u http://pd1.pingcap.com:2379  
Success!
```

## Related Articles

----------------

[OnCall-2633](https://internal.pingcap.net/jira/browse/ONCALL-2633?focusedCommentId=84482&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-84482): Streak - TiKV corruption/checksum crashes

[PD-3552](https://github.com/tikv/pd/issues/3552): PD should consider the available storage size to detect low space store

### Applies To

----------

All TiDB releases

#### Meta

**#timeout,** **#low-space-ratio,** **#stability, #administration**