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Archival Storage, SSD & Memory


Archival Storage is a solution to decouple growing storage capacity from compute capacity. Nodes with higher density and less expensive storage with low compute power are becoming available and can be used as cold storage in the clusters. Based on policy the data from hot can be moved to the cold. Adding more nodes to the cold storage can grow the storage independent of the compute capacity in the cluster.

The frameworks provided by Heterogeneous Storage and Archival Storage generalizes the HDFS architecture to include other kinds of storage media including SSD and memory. Users may choose to store their data in SSD or memory for a better performance.

Storage Types and Storage Policies

Storage Types: ARCHIVE, DISK, SSD and RAM_DISK

The first phase of Heterogeneous Storage (HDFS-2832) changed datanode storage model from a single storage, which may correspond to multiple physical storage medias, to a collection of storages with each storage corresponding to a physical storage media. It also added the notion of storage types, DISK and SSD, where DISK is the default storage type.

A new storage type ARCHIVE, which has high storage density (petabyte of storage) but little compute power, is added for supporting archival storage.

Another new storage type RAM_DISK is added for supporting writing single replica files in memory.

Storage Policies: Hot, Warm, Cold, All_SSD, One_SSD and Lazy_Persist

A new concept of storage policies is introduced in order to allow files to be stored in different storage types according to the storage policy.

We have the following storage policies:

  • Hot - for both storage and compute. The data that is popular and still being used for processing will stay in this policy. When a block is hot, all replicas are stored in DISK.
  • Cold - only for storage with limited compute. The data that is no longer being used, or data that needs to be archived is moved from hot storage to cold storage. When a block is cold, all replicas are stored in ARCHIVE.
  • Warm - partially hot and partially cold. When a block is warm, some of its replicas are stored in DISK and the remaining replicas are stored in ARCHIVE.
  • All_SSD - for storing all replicas in SSD.
  • One_SSD - for storing one of the replicas in SSD. The remaining replicas are stored in DISK.
  • Lazy_Persist - for writing blocks with single replica in memory. The replica is first written in RAM_DISK and then it is lazily persisted in DISK.

More formally, a storage policy consists of the following fields:

  1. Policy ID
  2. Policy name
  3. A list of storage types for block placement
  4. A list of fallback storage types for file creation
  5. A list of fallback storage types for replication

When there is enough space, block replicas are stored according to the storage type list specified in #3. When some of the storage types in list #3 are running out of space, the fallback storage type lists specified in #4 and #5 are used to replace the out-of-space storage types for file creation and replication, respectively.

The following is a typical storage policy table.

Policy ID Policy Name Block Placement (n  replicas) Fallback storages for creation Fallback storages for replication
15 Lazy_Persist RAM_DISK: 1, DISK: n-1 DISK DISK
7 Hot (default) DISK: n <none> ARCHIVE
2 Cold ARCHIVE: n <none> <none>

Note 1: The Lazy_Persist policy is useful only for single replica blocks. For blocks with more than one replicas, all the replicas will be written to DISK since writing only one of the replicas to RAM_DISK does not improve the overall performance.

Note 2: For the erasure coded files with striping layout, the suitable storage policies are All_SSD, Hot, Cold. So, if user sets the policy for striped EC files other than the mentioned policies, it will not follow that policy while creating or moving block.

Storage Policy Resolution

When a file or directory is created, its storage policy is unspecified. The storage policy can be specified using the “storagepolicies -setStoragePolicy” command. The effective storage policy of a file or directory is resolved by the following rules.

  1. If the file or directory is specified with a storage policy, return it.

  2. For an unspecified file or directory, if it is the root directory, return the default storage policy. Otherwise, return its parent’s effective storage policy.

The effective storage policy can be retrieved by the “storagepolicies -getStoragePolicy” command.


  • dfs.storage.policy.enabled - for enabling/disabling the storage policy feature. The default value is true.
  • dfs.datanode.data.dir - on each data node, the comma-separated storage locations should be tagged with their storage types. This allows storage policies to place the blocks on different storage types according to policy. For example:
    1. A datanode storage location /grid/dn/disk0 on DISK should be configured with [DISK]file:///grid/dn/disk0
    2. A datanode storage location /grid/dn/ssd0 on SSD can should configured with [SSD]file:///grid/dn/ssd0
    3. A datanode storage location /grid/dn/archive0 on ARCHIVE should be configured with [ARCHIVE]file:///grid/dn/archive0
    4. A datanode storage location /grid/dn/ram0 on RAM_DISK should be configured with [RAM_DISK]file:///grid/dn/ram0

    The default storage type of a datanode storage location will be DISK if it does not have a storage type tagged explicitly.

Mover - A New Data Migration Tool

A new data migration tool is added for archiving data. The tool is similar to Balancer. It periodically scans the files in HDFS to check if the block placement satisfies the storage policy. For the blocks violating the storage policy, it moves the replicas to a different storage type in order to fulfill the storage policy requirement. Note that it always tries to move block replicas within the same node whenever possible. If that is not possible (e.g. when a node doesn’t have the target storage type) then it will copy the block replicas to another node over the network.

  • Command:

    hdfs mover [-p <files/dirs> | -f <local file name>]
  • Arguments:

-p <files/dirs> Specify a space separated list of HDFS files/dirs to migrate.
-f <local file> Specify a local file containing a list of HDFS files/dirs to migrate.

Note that, when both -p and -f options are omitted, the default path is the root directory.

Storage Policy Commands

List Storage Policies

List out all the storage policies.

  • Command:

    hdfs storagepolicies -listPolicies
  • Arguments: none.

Set Storage Policy

Set a storage policy to a file or a directory.

  • Command:

    hdfs storagepolicies -setStoragePolicy -path <path> -policy <policy>
  • Arguments:

-path <path> The path referring to either a directory or a file.
-policy <policy> The name of the storage policy.

Unset Storage Policy

Unset a storage policy to a file or a directory. After the unset command the storage policy of the nearest ancestor will apply, and if there is no policy on any ancestor then the default storage policy will apply.

  • Command:

    hdfs storagepolicies -unsetStoragePolicy -path <path>
  • Arguments:

-path <path> The path referring to either a directory or a file.

Get Storage Policy

Get the storage policy of a file or a directory.

  • Command:

    hdfs storagepolicies -getStoragePolicy -path <path>
  • Arguments:

-path <path> The path referring to either a directory or a file.