Preamble

As part of a range deletion:

1. A batch of documents belonging to the orphaned range is deleted (each document in the batch is deleted in a single delete).
2. The counter of orphans is updated accordingly, triggering also an update of the orphans counter on the BalancerStatsRegistry.

Note that 1-2 are not happening in the same storage transaction, and - even if they would - the update of the in-memory stats registry would still not happen atomically.

The balancer is considering the cluster balanced if the following condition is met:

size(most loaded shard) <= size(least loaded shard) + 2 * chunkSize

Problem

If the range deletion happens on an already balanced collection while a range deletion is ongoing, it may happen that the balancer retrieves statistics from shards between bullets 1 and 2. This may trigger an unnecessary migration (no big deal as the chunk would be later migrated back, but still it's an unnecessary operation).

Example

Given a chunk size of 128MB for a given collection, if the collection size between between two shards A and B differs less than 256MB the balancer should not move any data.

However, it could happen the following:

• ShardA has 400MB, ShardB has 0MB
• Move a range of 128MB from A to B
• Move a range of 128MB from A to B
• ShardB receives a bit of inserts, let's say 24MB

At this point the collection is balanced because (280 - 144 = 136) is less than (2 * chunkSize = 256) :

• ShardA still has 400MB: 144MB (actual data size) plus 256MB (orphans)
• ShardB has 280MB (a bit more of 256MB, accounting also for some inserts)

The range deleter kicks in:

• Starts deleting the orphans for one range...
• The range deleter has so far deleted 125MB
• The balancer asks all the shards what's their data size:
  • Shard A will reply (400 [original size] - 125 [docs already deleted]) - (128 * 2 [tracked number of orphaned docs])
    So it replies 400 - 125 - 256 = 19MB
  • Shard B will reply 280MB
• The balancer performs its calculation on the chunkSize: (280 - 19 > 2 * chunkSize) ? YES because (261 > 256)
• A range is wrongly moved from shard B to shard A