Adding persistence volume is very much similar to adding a virtual hard drive to your machine instance. In Amazon EC2, an EBS storage device can be added as a persistence volume to the instance. For more information, see Attaching an Amazon EBS Volume to an Instance. Persistence volume capability comes handy when you want to store your content in a separate place and make the data available even after the instance is terminated or deleted. MySQL, MongoDB are examples where you may need this capability.
In most IaaSs such as EC2, one persistence volume can be attached to only one instance. Therefore if your deployment policy has
maxInstance count more than 1, you might encounter problems when Stratos tries to attach the volume to the second instance. Currently persistence volume mapping has been fully tested only in EC2 for Apache Stratos.| Table of Contents | ||
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Adding the persistence volume in the cartridge definition
The persistence volume is an optional configuration that can be specified by the user, if the user wants additional volumes to store content. Add a configuration similar to the following in the cartridge definition.
| Code Block |
|---|
"persistence": {
"isRequired": "true",
"volume" : [
{
"device": "/dev/sdc",
"mappingPath": "/home/ubuntu/sdc",
"size": "10",
"removeOnTermination": "false"
}
]
} |
Mapping multiple persistence volumes
Stratos provides the capability to specify multiple persistence volumes in the cartridge definition file as follows, in case you require more than one additional volume.
| Code Block |
|---|
"persistence": {
"isRequired": "true",
"volume" : [
{
"device": "/dev/sdc",
"mappingPath": "/home/ubuntu/sdc",
"size": "10",
"removeOnTermination": "false"
},
{
"device": "/dev/sdf",
"mappingPath": "/home/ubuntu/sdf",
"size": "20",
"removeOnTermination": "false"
}
]
} |
| Info |
|---|
You should not specify the same |
What happens behind the scene
When working with single tenant cartridges, persistence mapping will be enabled when the application is deployed and automatic subscription takes place. However, when working with multi-tenant cartridges, persistence mapping will be enabled during the tenant signup process.
Overall when persistence mapping is enabled Stratos will:
- Create the volumes.
- Format and create a writable file system (ext3) .
- Mount volumes to the directory specified as
mappingPath.
| Info |
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The process of formatting and creating a file system takes place only once when persistence mapping is enabled for the very first time. When the volume is being attached to another instance, after the first instance is terminated, it will not need to create a file system since the file system already exists. |
Verifying volume creation
Follow the instructions below to verify if the volumes have been created:
- Log in to the instance created via ssh.
Execute the command:
df -h
You will see an output as follows:Code Block df -h Filesystem Size Used Avail Use% Mounted on /dev/xvdc 10G 7.1G 97G 3% /home/ubuntu/sdc /dev/sda1 4000G 40G 320G 10% /home