A container is typically sharded when it has grown to have an object
count of shard_container_threshold + N, where N <<
shard_container_threshold. If sharded using the default
rows_per_shard of shard_container_threshold / 2 then this would
previously result in 3 shards: the tail shard would typically be
small, having only N rows. This behaviour caused more shards to be
generated than desirable.
This patch adds a minimum-shard-size option to
swift-manage-shard-ranges, and a corresponding option in the sharder
config, which can be used to avoid small tail shards. If set to
greater than one then the final shard range may be extended to more
than rows_per_shard in order to avoid a further shard range with less
than minimum-shard-size rows. In the example given, if
minimum-shard-size is set to M > N then the container would shard into
two shards having rows_per_shard rows and rows_per_shard + N
respectively.
The default value for minimum-shard-size is rows_per_shard // 5. If
all options have their default values this results in
minimum-shard-size being 100000.
Closes-Bug: #1928370
Co-Authored-By: Matthew Oliver <matt@oliver.net.au>
Change-Id: I3baa278c6eaf488e3f390a936eebbec13f2c3e55
2a593174a5