Keep-balance¶

Keep-balance enforces policies and generates reports about storage resource usage. It interacts with the Keep server and the metadata database (through the API server). Clients/users do not interact with keep-balance directly.

See also:

• Keep server
• Keep manifest format
• source:services/keep-balance
• http://doc.arvados.org/install/install-keep-balance.html

Responsibilities:

• Garbage collector: decide what is eligible for deletion (and some partial order of preference)
• Replication enforcer: copy and delete blocks in various backing stores to achieve desired replication level
• Rebalancer: move blocks to redistribute free space and reduce client probes

Example reports/queries:

• for managers: how much disk space is being conserved due to CAS
• for managers: how much disk space is occupied in a given backing store service
• for managers: how disk usage would be affected by modifying storage policy
• for managers: how much disk space+time is used (per user, group, node, disk)
• for users: when replication/policy specified for a collection is not currently satisfied (and why, for how long, etc)
• for users: how much disk space is represented by a given set of collections
• for users: how much disk space can be made available by garbage collection
• for users: how soon they should expect their cached data to disappear
• for users: performance statistics (how fast should I expect my job to read data?)
• for ops: where each block was most recently read/written, in case data recovery is needed
• for ops: how unbalanced the backing stores are across the cluster
• for ops: activity level and performance statistics
• for ops: activity level vs. amount of space (how much of the data is being accessed by users?)
• for ops: disk performance/error/status trends (and SMART reports) to help identify bad hardware
• for ops: history of disk adds, removals, moves

Basic kinds of data in the index:

• Which blocks are used by which collections (and which collections are valued by which users/groups)
• Which blocks are stored in which services (local Keep, remote Keep, other storage service)
• Which blocks are stored on which disks
• Which disks are attached to which nodes
• Aggregate read/write activity per block and per disk (where applicable, e.g., block stored in local Keep)
• Exceptions (checksum mismatch, IO error)

Implementation considerations¶

Overview

• REST service for queries
  • All requests require authentication. Token validity verified against Metadata server, and cached locally.
• Subscribes to system event log
• Connects to metadata server (has a system_user token), at least periodically, to ensure eventual consistency with metadata DB's idea of what data is important
• Persistent database
• In-memory database

Distributed/asynchronous

• Easy to run multiple keep index services.
• Most features do not need synchronous operation / real time data.
• Features that move or delete data should be tied to a single "primary" indexing service (failover event likely requires resetting some state).
• Substantial disagreement between multiple index services should be easy to flag on admin dashboard.