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Multi-pass mode to reduce keep-balance memory footprint » History » Revision 3

Revision 2 (Tom Clegg, 12/03/2024 03:53 PM) → Revision 3/4 (Tom Clegg, 01/16/2025 04:29 PM)

h1. 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 
 # run keep-balance without keeping the entire list of stored blocks in memory at once (addressed below) 
 # 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. 

 <pre><code class="yaml"> 
 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 
 </code></pre> 

 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.