Previously a shard might be shrunk if its object_count was fell below
the shrink_threshold. However, it is possible that a shard with few
objects has a large number of tombstones, which would result in a
larger than anticipated replication of rows to the acceptor shard.
With this patch, a shard's row count (i.e. the sum of tombstones and
objects) must be below the shrink_threshold before the shard will be
considered for shrinking.
A number of changes are made to enable tombstone count to be used in
shrinking decisions:
- DatabaseBroker reclaim is enhanced to count remaining tombstones
after rows have been reclaimed. A new TombstoneReclaimer class is
added to encapsulate the reclaim process and tombstone count.
- ShardRange has new 'tombstones' and 'row_count' attributes.
- A 'tombstones' column is added to the Containerbroker shard_range
table.
- The sharder performs a reclaim prior to reporting shard container
stats to the root container so that the tombstone count can be
included.
- The sharder uses 'row_count' rather than 'object_count' when
evaluating if a shard range is a shrink candidate.
Change-Id: I41b86c19c243220b7f1c01c6ecee52835de972b6
bcecddd517