Insert-only SyncWrite workload does not correctly trigger auto-compaction

Summary

The couchstore fragmentation calculation is not taking into account completed Prepares. As such, auto-compaction is not run when expected, and hence the completed (no longer needed) prepares are not purged.

Details

Using pilowfight to load 4M documents into two buckets; one using level=none, one with level=majority (same eviction policy, same compaction threshold of 10%):

cbc-pillowfight -U 127.0.0.1:9000/default -u Administrator -P asdasd --num-items=4000000 -m 4096 -M 4096 --random-body --populate-only --num-threads=10

cbc-pillowfight -U 127.0.0.1:9000/majority -u Administrator -P asdasd --num-items=4000000 -m 4096 -M 4096 --random-body --populate-only --num-threads=20 --durability=majority

(Note I also ran with a reduced vBucket count of 4; so make it easier to load a large number of documents (1M) per vBucket.)

This in the level=majority load having 2x the disk space:

• level=none

  $ couch_dbinfo --local default/0.couch.1

  DB Info (default/0.couch.1) - total disk size: 4.007 GB

  crc: CRC-32C

  Header at file offset 4302372864

  file format version: 13

  update_seq: 1000000

  purge_seq: 0

  timestamp: 1970-01-19T14:30:05.060290+01:00

  doc count: 1000000

  deleted doc count: 0

  data size: 3.885 GB

  B-tree size: 58.38 MB

  └── by-id tree: 27.38 MB

  └── by-seqno tree: 31.00 MB

  └── local size: 429 bytes

• level=majority

  $ couch_dbinfo --local majority/0.couch.2

  DB Info (majority/0.couch.2) - total disk size: 8.017 GB

  crc: CRC-32C

  Header at file offset 8608538624

  file format version: 13

  update_seq: 2000000

  purge_seq: 0

  timestamp: 1970-01-19T14:30:05.854591+01:00

  doc count: 2000000

  deleted doc count: 0

  data size: 7.784 GB

  B-tree size: 131.16 MB

  └── by-id tree: 62.12 MB

  └── by-seqno tree: 69.04 MB

  └── local size: 436 bytes

Interestingly the fragmentation percentage (measured as (couch_docs_actual_data_size - couch_docs_data_size) / couch_docs_actual_data_size is around 3%. However, if manually run compaction on the "majority" bucket (via the UI) the disk space shrinks to almost half:

$ couch_dbinfo --local majority/0.couch.3 

DB Info (majority/0.couch.3) - total disk size: 3.894 GB

   crc: CRC-32C

Header at file offset 4181381120

   file format version: 13

   update_seq: 2000000

   purge_seq: 0

   timestamp: 1970-01-19T14:30:05.854591+01:00

   doc count: 1000000

   deleted doc count: 0

   data size: 3.894 GB

   B-tree size:       68.07 MB

   └── by-id tree:    33.96 MB

   └── by-seqno tree: 34.11 MB

   └── local size:    434 bytes

What appears to be happening here is that the fragmentation calculation is incorrect - the on-disk Prepares (which have all been committed) are not counted as "overhead", and are instead treated as "valid" documents. This means auto-compaction hasn't run when it would be expected to. When it does run, however, these prepares can all be discarded and hence the file size after compaction is similar to the level=None case.