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Docs: Explained why not use AWS instance store for DB storage

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docs/source/appendices/example-rethinkdb-storage-setups.md
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## Example 1: A Partition of an AWS Instance Store
Many [AWS EC2 instance types](https://aws.amazon.com/ec2/instance-types/) comes with an [instance store](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/InstanceStorage.html): temporary storage that disappears when the instance disappears. (Some instance types _don't_ come with an instance store, but you can attach EBS storage.) The size and setup of an instance store depends on the EC2 instance type.
Many [AWS EC2 instance types](https://aws.amazon.com/ec2/instance-types/) comes with an [instance store](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/InstanceStorage.html): temporary storage that disappears when the instance disappears. The size and setup of an instance store depends on the EC2 instance type.
We have some scripts for [deploying a _test_ BigchainDB cluster on AWS](../clusters-feds/deploy-on-aws.html). Those scripts include commands to set up a partition (`/dev/xvdb`) on the [instance store](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/InstanceStorage.html) if that partition exists. Those commands can be found in the file `/deploy-cluster-aws/fabfile.py`, under `def install_rethinkdb()` (i.e. the function to install RethinkDB).
We have some scripts for [deploying a _test_ BigchainDB cluster on AWS](../clusters-feds/deploy-on-aws.html). Those scripts include commands to set up a partition (`/dev/xvdb`) on an [instance store](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/InstanceStorage.html) for RethinkDB data. Those commands can be found in the file `/deploy-cluster-aws/fabfile.py`, under `def install_rethinkdb()` (i.e. the Fabric function to install RethinkDB).
An AWS instance store is convenient, but it's intended for "buffers, caches, scratch data, and other temporary content." Moreover:
TODO: Discuss if/when one would use the instance store to store RethinkDB data.
* You pay for all the storage, regardless of how much you use.
* You can't increase the size of the instance store.
* If the instance stops, terminates, or reboots, you lose the associated instance store.
* Instance store data isn't replicated, so if the underlying disk drive fails, you lose the data in the instance store.
* "You can't detach an instance store volume from one instance and attach it to a different instance."
The [AWS documentation says](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/InstanceStorage.html), "...do not rely on instance store for valuable, long-term data. Instead, you can build a degree of redundancy (for example, RAID 1/5/6), or use a file system (for example, HDFS and MapR-FS) that supports redundancy and fault tolerance."
**Even if you don't use an AWS instance store partition to store your node's RethinkDB data, you may find it useful to read the steps in `def install_rethinkdb()`: [see fabfile.py](https://github.com/bigchaindb/bigchaindb/blob/master/deploy-cluster-aws/fabfile.py).**
## Example 2: An Amazon EBS Volume
Amazon EBS volumes are always replicated.
TODO
Note: Amazon EBS volumes are always replicated.
## Example 3: Using Amazon EFS
@ -21,7 +31,8 @@ Amazon EBS volumes are always replicated.
TODO
## Example 4: A RAID Example?
## Other Examples?
Maybe make two EBS volumes look like one?
TODO
Maybe RAID, ZFS, HDFS, MapR-FS, ... (over EBS volumes, i.e. a DIY Amazon EFS)