kocolosk commented on issue #1065: [Question] Periodic performance degradation
URL: https://github.com/apache/couchdb/issues/1065#issuecomment-351898153
 
 
   A very fair question.
   
   It turns out that compaction cleans up more than just old document 
revisions. Any time a document is written CouchDB needs to update a pair of 
Btree indexes that help to quickly locate the document on disk. Like everything 
else in CouchDB, these indexes are maintained in a purely copy-on-write 
fashion. This means that the database file accumulates a lot of old Btree 
nodes, even when the database is only seeing new inserts.  Eventually the 
volume of old Btree nodes alone is enough to trip the db_fragmentation 
threshold.

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