Add Compaction Logging

[tonysun83]

To predict future compaction results, we log pre-compaction and
post-compaction file sizes. These log results will be used as data
points for regression analysis.

COUCHDB-3417

Overview

Users sometimes want to know the results of compaction and how much disk space will be freed up. Unfortunately, using simply data size and disk size to calculate this prediction is misleading due to conflicts and attachments. This approach logs disk size pre and post compaction for db and view shard files. The hope is that with enough data points, users can run regression analysis to heuristically determine a compaction result.

Testing recommendations

To see the output, one can connect to a node and run compaction on db or view to see the log result.
{ok, Db} = couch_db:open(<<"shards/00000000-1fffffff/mydb.1494471596">>, []).
couch_db:start_compact(Db).

JIRA issue number

COUCHDB-3417

Related Pull Requests

Checklist

• Code is written and works correctly;
• Changes are covered by tests;
• Documentation reflects the changes;
• I will not forget to update rebar.config.script
  with the correct commit hash once this PR get merged.

[davisp]

Three tweaks:

Don't report the file name twice, the extra copy with .compact appended isn't going to be useful.
Change your format to something like "Compaction swap for db ~s ~p ~p" so that its easier to parse.
Also might want to bump this to notice level since it seems more important than info.

[tonysun83]

@davisp : made the changes you suggested

[notice] 2017-05-11T16:56:25.382380Z [email protected] <0.1945.0> -------- Compaction swap for view /Users/tonysun/github/apache/couchdb/dev/lib/node1/data/.shards/00000000-1fffffff/testdb.1494473014_design/mrview/f39a9b2d4ad85d129c9a9b6d0018bb36.view 8296 4200

[notice] 2017-05-11T16:59:19.584582Z [email protected] <0.826.0> -------- Compaction swap for db: /Users/tonysun/github/apache/couchdb/dev/lib/node1/data/shards/00000000-1fffffff/mydb.1494471596.couch 49305 16537

[davisp]

+1

[davisp]

@tonysun83 Any reason you haven't merged this?

[tonysun83]

@davisp: wups got lost in the clutter, will do that today