What should I do when one of namespace SSDs fails? Does the aerospike copy the data across SSDs associated with a namespace? How to replace failed SSD and restore the data?
This does depend on your configurations. Aerospike does allow the possibility to have an unreplicated namespace.
Normally, you should have you replication factor to at least 2. Assuming you do this, the cluster will automatically copy the data between the remaining nodes. On Community Edition, you are limited to 2 nodes, so no redistribution is needed or really even possible. For Enterprise Edition (and Trial Edition) the data will be automatically redistributed. No commands are necessary to rebalance the data.
Let’s say that you have 3 SSDs on your nodes and one fails. You will obviously need to physically replace the failed SSD. Currently, Aerospike must either assume that all the data on all the SSDs are valid, or that there is no data. So in order to make use of a new drive, you must clear ALL the SSDs on the failed nodes by re-initializing the drive. Instructions for this can be found in the documentation:
https://docs.aerospike.com/display/AS2/ … ialization