Tom Clegg, 04/10/2013 10:21 PM
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 a cryptographic 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 cryptographic digest and 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
The responsibilities of management agents are:
* Verify validity of permission tokens
* Determine which blocks have higher or lower redundancy than required
* Monitor disk space and move or delete blocks as needed
* Collect per-user, per-group, per-node, and per-disk usage statistics