Magma not observing fragmentation threshold

My 50MB (uncompressed) dataset took over 500M of disk space despite having the default 50% fragmentation threshold specified. Only after fully stopping the workload for several minutes was the disk space reclaimed (bottomed out at around 160M).

Logs here: s3://cb-customers-secure/davidh/2023-02-03/collectinfo-2023-02-03t174442-ns_1@127.0.0.1.zip

Setup

• Single 7.1.3 node with one Magma bucket and 2 collections.
• Pillowfight command to populate 50M worth of data as 100,000 x 512 byte JSON docs at 200 ops / sec

  cbc-pillowfight -U couchbase://10.145.233.101/charlie_magma --collection _default.incoming -m 512 -M 512 -J -p DH_ -I 100000 -t1 -r100 -u admin -P password --rate-limit 200

• Eventing function to move the data set from one collection to another:

  function OnUpdate(doc, meta) {

  dest[meta.id]=doc

  delete source[meta.id]

  }

Observations
The disk space used by magma far exceeded the total size of the data set, allowing the fragmentation to greatly exceed the specified level of 50%.
Only completely halting the workload allowed the disk space to come back under control.