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In MMAP items in a capped collection cannot grow - as per the documentation and an error is raised if they do.

> db.createCollection("cap",{capped:true,size:128000})

{ "ok" : 1 }

>  db.cap.update({_id:1},{$set:{test:"hello",vals:["one"]}},true,true)

WriteResult({ "nMatched" : 0, "nUpserted" : 1, "nModified" : 0, "_id" : 1 })

>  db.cap.update({_id:1},{$set:{test:"hello"},$push:{vals:"two"}},true,true)

WriteResult({

	"nMatched" : 0,

	"nUpserted" : 0,

	"nModified" : 0,

	"writeError" : {

		"code" : 10003,

		"errmsg" : "failing update: objects in a capped ns cannot grow"

})

In WiredTiger they are allowed to grow - is this deliberate?

> db.createCollection("cap",{capped:true,size:128000})

{ "ok" : 1 }

> db.cap.update({_id:1},{$set:{test:"hello",vals:["one"]}},true,true)

WriteResult({ "nMatched" : 0, "nUpserted" : 1, "nModified" : 0, "_id" : 1 })

> db.cap.findOne()

{ "_id" : 1, "test" : "hello", "vals" : [ "one" ] }

> db.cap.update({_id:1},{$set:{test:"hello"},$push:{vals:"two"}},true,true)

WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })

> db.cap.findOne()

{ "_id" : 1, "test" : "hello", "vals" : [ "one", "two" ] }

is documented by

DOCS-8961 [Server] WiredTiger allows capped collection objects to grow

is duplicated by

SERVER-23981 Incompatible to update capped collection after upgrade to MongoDB-3.2

is related to

SERVER-6984 Initial sync can fail, or break future replication, when updates shrink or grow docs in capped collections

SERVER-11983 Update on document without _id, in capped collection without _id index, creates an _id field

SERVER-14115 cannot perform an in-place update on oplog

SERVER-19551 Keep "milestones" against WT oplog to efficiently remove old records

related to

SERVER-58865 Remove obsolete restriction on capped collection objects

(1 is related to, 1 related to)

Participants:: Akira Kurogane, Asya Kamsky, Daniel Pasette, David Hows, Geert Bosch, Githook User, John Page