persistent-volumes.md

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Multiple Disks

Mesos provides a mechanism for operators to expose multiple disk resources. When creating persistent volumes frameworks can decide whether to use specific disks by examining the source field on the disk resources offered.

Disk resources come in three forms:

• A Root disk is presented by not having the source set in DiskInfo.
• A Path disk is presented by having the PATH enum set for source in DiskInfo. It also has a root which the operator uses to specify the directory to be used to store data.
• A Mount disk is presented by having the MOUNT enum set for source in DiskInfo. It also has a root which the operator uses to specify the mount point used to store data.

Operators can use the JSON-formated --resources option on the agent to provide these different kind of disk resources on agent start-up.

Root disk

A Root disk is the basic disk resource in Mesos. It usually maps to the storage on the main operating system drive that the operator has presented to the agent. Data is mapped into the work_dir of the agent.

    {
      "resources" : [
        {
          "name" : "disk",
          "type" : "SCALAR",
          "scalar" : { "value" : 2048 },
          "role" : <framework_role>
        }
      ]
    }

Path disks

A Path disk is an auxiliary disk resource provided by the operator. This can can be carved up into smaller chunks by accepting less than the total amount as frameworks see fit. Common uses for this kind of disk are extra logging space, file archives or caches, or other non performance-critical applications. Operators can present extra disks on their agents as Path disks simply by creating a directory and making that the root of the Path in DiskInfo's source.

Path disks are also useful for mocking up a multiple disk environment by creating some directories on the operating system drive. This should only be done in a testing or staging environment.

    {
      "resources" : [
        {
          "name" : "disk",
          "type" : "SCALAR",
          "scalar" : { "value" : 2048 },
          "role" : <framework_role>,
          "disk" : {
            "source" : {
              "type" : "PATH",
              "path" : { "root" : "/mnt/data" }
            }
          }
        }
      ]
    }

Mount disks

A Mount disk is an auxiliary disk resource provided by the operator. This cannot be carved up into smaller chunks by frameworks. This lack of flexibility allows operators to provide assurances to frameworks that they will have exclusive access to the disk device. Common uses for this kind of disk include database storage, write-ahead logs, or other performance-critical applications.

Another requirement of Mount disks is that (on Linux) they map to a mount point in the /proc/mounts table. Operators should mount a physical disk with their preferred file system and provide the mount point as the root of the Mount in DiskInfo's source.

Aside from the performance advantages of Mount disks, applications running on them should be able to rely on disk errors when they attempt to exceed the capacity of the volume. This holds true as long as the file system in use correctly propagates these errors. Due to this expectation, the posix/disk isolation is disabled for Mount disks.

    {
      "resources" : [
        {
          "name" : "disk",
          "type" : "SCALAR",
          "scalar" : { "value" : 2048 },
          "role" : <framework_role>,
          "disk" : {
            "source" : {
              "type" : "MOUNT",
              "mount" : { "root" : "/mnt/data" }
            }
          }
        }
      ]
    }

Block disks

Mesos currently does not allow operators to expose raw block devices. It may do so in the future, but there are security and flexibility concerns that need to be addressed in a design document first.

Storage Management

Mesos currently does not clean up or destroy data when persistent volumes are destroyed. It may do so in the future; however, the expectation is currently upon the framework, executor, and application to delete their data before destroying their persistent volumes. This is strongly encouraged for both security and ensuring that future users of the underlying disk resource are not penalized due to prior consumption of the disk capacity.

Implementation

A Path disk will have sub-directories created within the root which will be used to differentiate the different volumes that are created on it.

A Mount disk will not have sub-directories created, allowing applications to use the full file system mounted on the device. This construct allows Mesos tasks to access volumes that contain pre-existing directory structures. This can be useful to simplify ingesting data such as a pre-existing Postgres database or HDFS data directory.

Operators should be aware of these distinctions when inspecting or cleaning up remnant data.