Why has my newly created mongodb local database grown to 24GB?

Question

I setup a mongodb replica set a few days ago, I did some little test on it and everything working well. Today I just found its local collection grew to 24G !!!

rs0:PRIMARY> show dbs
local   24.06640625GB
test    0.203125GB

The other collections look normal except "oplog.rs":

rs0:PRIMARY> db.oplog.rs.stats()
{
    "ns" : "local.oplog.rs",
    "count" : 322156,
    "size" : 119881336,
    "avgObjSize" : 372.12200300475547,
    "storageSize" : 24681987920,
    "numExtents" : 12,
    "nindexes" : 0,
    "lastExtentSize" : 1071292416,
    "paddingFactor" : 1,
    "systemFlags" : 0,
    "userFlags" : 0,
    "totalIndexSize" : 0,
    "indexSizes" : {

    },
    "capped" : true,
    "max" : NumberLong("9223372036854775807"),
    "ok" : 1
}

This is my mongodb.conf

dbpath=/data/db

#where to log
logpath=/var/log/mongodb/mongodb.log
logappend=true
port = 27017
fork = true
replSet = rs0

How can I solve it? Many thanks.

Answer 1

The oplog, which keeps an ongoing log for the operations on the primary and is used for the replication mechanism, is allocated by default at 5% of the available free disk space (for Linux/Unix systems, not Mac OS X or Windows). So if you have a lot of free disk space, MongoDB will make a large oplog, which means you have a large time window within which you could restore back to any point in time, for instance. Once the oplog reaches its maximum size, it simply rolls over (ring buffer).

You can specify the size of the oplog when initializing the database using the oplogSize option, see http://docs.mongodb.org/manual/core/replica-set-oplog/

Bottom line: Unless you are really short of disk space (which apparently you aren't, otherwise the oplog wouldn't have been created so big), don't worry about it. It provides extra security.