Multi-pass mode to reduce keep-balance memory footprint¶

Background: Currently keep-balance's RAM footprint increases with the number of stored blocks. On a large site, even the largest available machine might not have enough RAM to complete a keep-balance cycle. Scalability will be vastly improved if we can

1. run keep-balance without keeping the entire list of stored blocks in memory at once (addressed below)
2. distribute the keep-balance work across multiple system hosts or compute nodes (future work)

Proposal: On large clusters, balance in N passes where each pass considers 1/N of the possible block locators -- for example, if N=16, the first pass considers blocks whose locators begin with "0", the next pass "1", etc. For simplicity, N must be a power of 16.

Clusters:
  xxxxx:
    Collections:
      # When rebalancing, split the stored blocks into the specified
      # number of bins and process one bin at a time. The default (1)
      # is suitable for small clusters. Larger numbers (16, 256) are
      # needed when the keep-balance host does not have enough RAM to
      # hold the entire list of block IDs.
      #
      # BalanceBins must be a power of 16.
      BalanceBins: 1

New behaviors relating to trash/pull lists:

• When starting a new sweep, clear trash lists (no change to existing behavior)
• [keepstore] When posting a new trash/pull list, check X-Keep-List-Prefix header, and don't clear existing entries that have a different prefix
• [keep-balance] When posting a new trash/pull list, set X-Keep-List-Prefix header, so keepstore knows which entries to clear
• [keep-balance] Run a pass for each prefix, then merge resulting statistics to produce full cluster summary

New behaviors relating to setting replication_confirmed:

• [rails] add collections column replication_confirmed_partial, default null
• [rails] reset replication_confirmed_partial=null when updating a collection (just like existing behavior of replication_confirmed)
• [keep-balance] when starting a multi-pass sweep, clear replication_confirmed_partial:
  • update collections set replication_confirmed_partial=NULL
• [keep-balance] after each pass (single prefix), set or reduce replication_confirmed_partial:
  • update collections set replication_confirmed_partial=min($1,coalesce(replication_confirmed_partial,$1)) where portable_data_hash=$2
• [keep-balance] after all passes (prefixes) are done, copy replication_confirmed_partial to replication_confirmed:
  • update collections set replication_confirmed=replication_confirmed_partial, replication_confirmed_at=$1 where replication_confirmed_partial is not NULL

Concurrency:

• For now, a single keep-balance process will perform N passes serially, then merge results.
• In future, we should allow multiple keep-balance processes on different nodes running passes concurrently. This will require further coordination such that a single "coordinator" process merges statistics produced by "worker" processes and updates replication_confirmed when all workers are finished. Ideally, the workers can be automatically dispatched as containers on cloud/HPC nodes.