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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.

Types of Disk Resources

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. Example resource values in JSON format can be found below. By default (if --resources is not specified), the Mesos agent will only make the root disk available to the cluster.

NOTE: Once you specify any Disk resource manually (i.e., via the --resources flag), Mesos will stop auto-detecting the Root disk resource. Hence if you want to use the Root disk you will need to manually specify it using the format described below.

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.

An example resources value for a root disk is shown below. Note that the operator could optionally specify a role for the disk, which would result in statically reserving the disk for a single role.

    {
      "resources" : [
        {
          "name" : "disk",
          "type" : "SCALAR",
          "scalar" : { "value" : 2048 }
        }
      ]
    }

Path disks

A Path disk is an auxiliary disk resource provided by the operator. This can be carved up into smaller chunks by creating persistent volumes that use less than the total available space on the disk. 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. Note that creating multiple Path disks on the same filesystem requires statically partitioning the available disk space. For example, suppose a 10GB storage device is mounted to /foo and the Mesos agent is configured with two Path disks at /foo/disk1 and /foo/disk2. To avoid the risk of running out of space on the device, disk1 and disk2 should be configured (when the Mesos agent is started) to use at most 10GB of disk space in total.

An example resources value for a Path disk is shown below. Note that the operator could optionally specify a role for the disk, which would result in statically reserving the disk for a single role.

    {
      "resources" : [
        {
          "name" : "disk",
          "type" : "SCALAR",
          "scalar" : { "value" : 2048 },
          "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.

On Linux, Mount disks must 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 disk/du isolation is disabled for Mount disks.

An example resources value for a Mount disk is shown below. Note that the operator could optionally specify a role for the disk, which would result in statically reserving the disk for a single role.

    {
      "resources" : [
        {
          "name" : "disk",
          "type" : "SCALAR",
          "scalar" : { "value" : 2048 },
          "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.

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. When a persistent volume on a Path disk is destroyed, Mesos will remove all the files and directories stored in the volume, as well as the sub-directory within root that was created by Mesos for the volume.

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. Note that when a persistent volume on a Mount disk is destroyed, Mesos will remove all the files and directories stored in the volume, but will not remove the root directory (i.e., the mount point).

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