Type: New Feature
Affects Version/s: None
Fix Version/s: None
In HDFS-7240, Ozone defines storage containers to store both the data and the metadata. The storage container layer provides an object storage interface and aims to manage data/metadata in a distributed manner. More details about storage containers can be found in the design doc in HDFS-7240.
HDFS can adopt the storage containers to store and manage blocks. The general idea is:
- Each block can be treated as an object and the block ID is the object's key.
- Blocks will still be stored in DataNodes but as objects in storage containers.
- The block management work can be separated out of the NameNode and will be handled by the storage container layer in a more distributed way. The NameNode will only manage the namespace (i.e., files and directories).
- For each file, the NameNode only needs to record a list of block IDs which are used as keys to obtain real data from storage containers.
- A new DFSClient implementation talks to both NameNode and the storage container layer to read/write.
HDFS, especially the NameNode, can get much better scalability from this design. Currently the NameNode's heaviest workload comes from the block management, which includes maintaining the block-DataNode mapping, receiving full/incremental block reports, tracking block states (under/over/miss replicated), and joining every writing pipeline protocol to guarantee the data consistency. These work bring high memory footprint and make NameNode suffer from GC. HDFS-5477 already proposes to convert BlockManager as a service. If we can build HDFS on top of the storage container layer, we not only separate out the BlockManager from the NameNode, but also replace it with a new distributed management scheme.
The storage container work is currently in progress in HDFS-7240, and the work proposed here is still in an experimental/exploring stage. We can do this experiment in a feature branch so that people with interests can be involved.
A design doc will be uploaded later explaining more details.