couchstore node flushing doesn't respect node quota size

Summary

When couchstore writes out modified node elements during document saves, it it supposed to limit the number of bytes written in a single node to a _chunk_threshold - by default 1279 bytes. If the node is larger than the limit it should be split into multiple sibling nodes.

However, this limit is not respected, resulting in overly-large nodes being written out. In the case of the by-seqno B-Tree (which always writes values to the rightmost leaf as seqnos are increasing), it results in all leaf elements residing in a single leaf node. Moreover, this means that adding another element to the B-Tree effectively re-writes the entire tree, resulting in massive Write Amplification.

Steps to Reproduce

1. Start 2-node cluster run (single replica):

   ./cluster_run --nodes=2

2. Start a SyncWrite workload; single threaded client updating the same 100,000 items (each key 10 times) with level=persistMajority:

   ./engines/ep/management/sync_repl.py localhost:12000 Administrator asdasd default loop_bulk_setD key value 100000 1000000 3

3. Observe the op/s and Write Amplification

Expected Results

• Op/s should be broadly constant (given both key and seqno B-Tree should have a constant number of items in them).
• Write amplification should also be broadly constant.

Actual Results

• The op/s quickly drops from a peak of ~600 down to 150:
• The Write Amplification increase (corresponding with the top in op/s) from 5.6x, up to 743x .
• Both op/s and Write Amplification temporarily recover when compaction occurs, but same pattern is observed over time: