Recovering from a single disk failure using JBOD

Steps for recovering from a single disk failure in a disk array using JBOD (just a bunch of disks).

DataStax Enterprise might not fail from the loss of one disk in a JBOD array, but some reads and writes may fail when:

The operation's consistency level is ALL.
The data being requested or written is stored on the defective disk.
The data to be compacted is on the defective disk.

It's possible that you can simply replace the disk, restart DataStax Enterprise, and run nodetool repair. However, if the disk crash corrupted system table, you must remove the incomplete data from the other disks in the array. The procedure for doing this depends on whether the cluster uses Vnode or single-token architecture.

cassandra.yaml

The location of the cassandra.yaml file depends on the type of installation:

Package installations	/etc/dse/cassandra/cassandra.yaml
Tarball installations	`installation_location`/resources/cassandra/conf/cassandra.yaml

cassandra-env.sh

The location of the cassandra-env.sh file depends on the type of installation:

Package installations	/etc/dse/cassandra/cassandra-env.sh
Tarball installations	`installation_location`/resources/cassandra/conf/cassandra-env.sh

Procedure

Verify that the node has a defective disk and identify the disk, by checking the logs on the affected node.
Disk failures are logged in FILE NOT FOUND entries, which identifies the mount point or disk that has failed.
If the node is still running, stop DSE and shut down the node.
Replace the defective disk and restart the node.
If the node cannot restart:
1. Try restarting DataStax Enterprise without bootstrapping the node:
  Package installations:
  1. Add the following option to cassandra-env.sh file:
```
JVM_OPTS="$JVM_OPTS -Dcassandra.allow_unsafe_replace=true
```
  2. Starting DataStax Enterprise as a service.
  3. After the node bootstraps, remove the -Dcassandra.allow_unsafe_replace=true parameter from cassandra-env.sh.
  4. Starting DataStax Enterprise as a service.
  Tarball installations:
  - Start DataStax Enterprise with this option:
```
sudo bin/dse cassandra Dcassandra.allow_unsafe_replace=true
```
    Tarball path:
```
installation_location
```
If DataStax Enterprise restarts, run nodetool repair on the node. If not, replace the node.
If the repair succeeds, the node is restored to production. Otherwise, go to 7 or 8.
For a cluster using vnodes:
1. On the affected node, clear the system directory on each functioning drive.
  Example for a node with a three disk JBOD array:
```
-/mnt1/cassandra/data 
-/mnt2/cassandra/data
-/mnt3/cassandra/data
```
  If mnt1 has failed:
```
rm -fr /mnt2/cassandra/data/system && 
rm -fr /mnt3/cassandra/data/system
```
2. Restart DataStax Enterprise without bootstrapping as described in 4:
```
-Dcassandra.allow_unsafe_replace=true
```
3. Run nodetool repair on the node.
  
  If the repair succeeds, the node is restored to production. If not, replace the dead node.
For a cluster single-token nodes:
1. On one of the cluster's working nodes, run nodetool ring to retrieve the list of the repaired node's tokens:
```
nodetool ring | grep ip_address_of_node | awk ' {print $NF ","}' | xargs
```
2. Copy the output of the nodetool ring into a spreadsheet (space-delimited).
3. Edit the output, keeping the list of tokens and deleting the other columns.
4. On the node with the new disk, open the cassandra.yaml file and add the tokens (as a comma-separated list) to the initial_token property.
5. Change any other non-default settings in the new nodes to match the existing nodes. Use the diff command to find and merge any differences between the nodes.
  
  If the repair succeeds, the node is restored to production. If not, replace the node.
6. On the affected node, clear the system directory on each functioning drive.
  Example for a node with a three disk JBOD array:
```
-/mnt1/cassandra/data 
-/mnt2/cassandra/data
-/mnt3/cassandra/data
```
  If mnt1 has failed:
```
rm -fr /mnt2/cassandra/data/system && 
rm -fr /mnt3/cassandra/data/system
```
7. Restart DataStax Enterprise without bootstrapping as described in 4:
```
-Dcassandra.allow_unsafe_replace=true
```
8. Run nodetool repair on the node.
  
  If the repair succeeds, the node is restored to production. If not, replace the node.