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:
Root
disk is presented by not having the source
set in DiskInfo
.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.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
diskA 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.
[ { "name" : "disk", "type" : "SCALAR", "scalar" : { "value" : 2048 } } ]
Path
disksA 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.
[ { "name" : "disk", "type" : "SCALAR", "scalar" : { "value" : 2048 }, "disk" : { "source" : { "type" : "PATH", "path" : { "root" : "/mnt/data" } } } } ]
Mount
disksA 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.
[ { "name" : "disk", "type" : "SCALAR", "scalar" : { "value" : 2048 }, "disk" : { "source" : { "type" : "MOUNT", "mount" : { "root" : "/mnt/data" } } } } ]
Block
disksMesos 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.
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.