Keep is a distributed content-addressable storage system designed for high performance in I/O-bound cluster environments.
Notable design goals and features include:
- High scalability
- Node-level redundancy
- Maximum overall throughput in a busy cluster environment
- Maximum data bandwidth from client to disk
- Minimum transaction overhead
- Elimination of disk thrashing (commonly caused by multiple simultaneous readers)
- Client-controlled redundancy
The above goals are accomplished by the following design features.
- Data is transferred directly between the client and the physical node where the disk is installed.
- Data collections are encoded in large (≤64 MiB) blocks to minimize short read/write operations.
- Each disk accepts only one block-read/write operation at a time. This prevents disk thrashing and maximizes total throughput when many clients compete for a disk.
- Storage redundancy is directly controlled, and can be easily verified, by the client simply by reading or writing a block of data on multiple nodes.
- Data block distribution is computed based on the MD5 digest of the data block being stored or retrieved. This eliminates the need for a central or synchronized database of block storage locations.
The Keep storage system consists of data block read/write services, SDKs, and management agents.
The responsibilities of the Keep service are:
- Write data blocks
- When writing: ensure data integrity by comparing client-supplied MD5 digest to client-supplied data
- Read data blocks (subject to permission, which is determined by the system/metadata DB)
- Send read/write/error event logs to management agents
The responsibilities of the SDK are:
- When writing: split data into ≤64 MiB chunks
- When writing: encode directory trees as manifests
- When writing: write data to the desired number of nodes to achieve storage redundancy
- After writing: register a collection with Arvados
- When reading: parse manifests
- When reading: verify data integrity by comparing locator to MD5 digest of retrieved data