Andrew Purtell created HDFS-4672:
------------------------------------
Summary: Support tiered storage policies
Key: HDFS-4672
URL: https://issues.apache.org/jira/browse/HDFS-4672
Project: Hadoop HDFS
Issue Type: New Feature
Components: datanode, hdfs-client, libhdfs, namenode
Reporter: Andrew Purtell
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.
--
This message is automatically generated by JIRA.
If you think it was sent incorrectly, please contact your JIRA administrators
For more information on JIRA, see: http://www.atlassian.com/software/jira
