Support tiered storage policies

We would like to be able to create certain files on certain storage device classes (e.g. spinning media, solid state devices, RAM disk, non-volatile memory). HDFS-2832 enables heterogeneous storage at the DataNode, so the NameNode can gain awareness of what different storage options are available in the pool and where they are located, but no API is provided for clients or block placement plugins to perform device aware block placement. We would like to propose a set of extensions that also have broad applicability to use cases where storage device affinity is important:

• Add an enum of generic storage device classes, borrowing from current taxonomy of the storage industry

• Augment DataNode volume metadata in storage reports with this enum

• Extend the namespace so pluggable block policies can be specified on a directory and storage device class can be tracked in the Inode. Perhaps this could be a larger discussion on adding support for extended attributes in the HDFS namespace. The Inode should track both the storage device class hint and the current actual storage device class. FileStatus should expose this information (or xattrs in general) to clients.

• Extend the pluggable block policy framework so policies can also consider, and specify, affinity for a particular storage device class

• Extend the file creation API to accept a storage device class affinity hint. Such a hint can be supplied directly as a parameter, or, if we are considering extended attribute support, then instead as one of a set of xattrs. The hint would be stored in the namespace and also used by the client to indicate to the NameNode/block placement policy/DataNode constraints on block placement. Furthermore, if xattrs or device storage class affinity hints are associated with directories, then the NameNode should provide the storage device affinity hint to the client in the create API response, so the client can provide the appropriate hint to DataNodes when writing new blocks.

• The list of candidate DataNodes for new blocks supplied by the NameNode to clients should be weighted/sorted by availability of the desired storage device class.

• Block replication should consider storage device affinity hints. If a client move()s a file from a location under a path with affinity hint X to under a path with affinity hint Y, then all blocks currently residing on media X should be eventually replicated onto media Y with the then excess replicas on media X deleted.

• Introduce the concept of degraded path: a path can be degraded if a block placement policy is forced to abandon a constraint in order to persist the block, when there may not be available space on the desired device class, or to maintain the minimum necessary replication factor. This concept is distinct from the corrupt path, where one or more blocks are missing. Paths in degraded state should be periodically reevaluated for re-replication.

• The FSShell should be extended with commands for changing the storage device class hint for a directory or file.

• Clients like DistCP which compare metadata should be extended to be aware of the storage device class hint. For DistCP specifically, there should be an option to ignore the storage device class hints, enabled by default.

Suggested semantics:

• The default storage device class should be the null class, or simply the “default class”, for all cases where a hint is not available. This should be configurable. hdfs-defaults.xml could provide the default as spinning media.

• A storage device class hint should be provided (and is necessary) only when the default is not sufficient.

• For backwards compatibility, any FSImage or edit log entry lacking a storage device class hint is interpreted as having affinity for the null class.

• All blocks for a given file share the same storage device class. If the replication factor for this file is increased the replicas should all be placed on the same storage device class.

• If one or more blocks for a given file cannot be placed on the required device class, then the file is marked as degraded. Files in degraded state should be periodically reevaluated for re-replication.

• A directory and path can only have one storage device affinity hint. If the file inode specifies a hint, this is used, otherwise we walk up the path until a hint is found and use that one, otherwise the default storage class is used.