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vpetersson
Asked: 2014-07-28 04:07:10 +0800 CST

Very strange disk-usage for replica in Mongodb

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tl;dr: the new 2.6 replica eats up a lot more disk space than the 2.4 members.

We're heavy users of MongoDB's GridFS. We're currently on 2.4, and are intending to upgrade to 2.6 by simply adding a new replica members and gradually deprecate the 2.4 nodes as recommended.

We have one primary, one secondary and an arbiter. Here are some data on the current system:

$ mongod --version
db version v2.4.10
Sun Jul 27 13:56:27.250 git version: e3d78955d181e475345ebd60053a4738a4c5268a

Here's some data on the database:

> db.stats()
{
    "db" : "SomeDatabase",
    "collections" : 4,
    "objects" : 2797931,
    "avgObjSize" : 284877.9334958582,
    "dataSize" : 797068801344,
    "storageSize" : 946689860768,
    "numExtents" : 469,
    "indexes" : 5,
    "indexSize" : 251248480,
    "fileSize" : 950804676608,
    "nsSizeMB" : 16,
    "dataFileVersion" : {
        "major" : 4,
        "minor" : 5
    },
    "ok" : 1
}

As you can see, the database is ~950GB and fits (but barely) on the Primary and Secondary (which is 1TB of storage).

Since we're running low on storage, I gave the new node that I wanted to add to the system another 200G (i.e. 1.2TB).

Here's the MongoDB version from the new node:

$ mongod --version
db version v2.6.3
2014-07-27T12:01:21.242+0000 git version: 255f67a66f9603c59380b2a389e386910bbb52cb

After making it a member of the replica set it starts to sync just fine (like every other time we've rotated ReplicaSet members). This is where it gets weird. After syncing for a few hours, it reaches the expected database size (~950GB). However, it just simply continues after that until it finally fills up the entire disk and crashes.

Now, I know that 2.6 is a new version, but how come that the database on disk exceeds the ones on the Primary and Secondary? Any pointers would be very helpful.

database

1 Answers

  1. In MongoDB 2.6 PowerOf2sizes (http://docs.mongodb.org/master/reference/command/collMod/#usePowerOf2Sizes) is the default setting for collections. It will cause fewer disk relocations during updates but will consume more disk space.

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