CODEWITHSUNDEEP
mongodb
manida
manida

Reputation: 41

How to force preallocation in MongoDB

I want to create prealloc.X files in my journal folder, there is no option to explicitly create these files, it is decided by mongo whether preallocation will be done or not.

I used --journal option but that didin't help.

Please suggest any way or condition where prealloc.X files can be created.

Thanks in advance!

Upvotes: 1

Views: 2095

Answers (2)

balafi
balafi

Reputation: 2153

Preallocation is done by default when journalling is enabled (enabled by default in V2.2 and on). In fact you need to explicitly tell mongo if you want to disable preallocation - using the noprealloc flag.

I'd suggest you first look if noprealloc is set to true in your mongodb.conf and change it to false.

But you can also manually create the prealloc.X even before mongod is started so to save the Preallocation Lag. This is described in detail here http://docs.mongodb.org/manual/administration/journaling/#avoid-preallocation-lag

The idea is you start mongod just to cause the prealloc.X files be created. Then you stop it and copy the files to your journal directory of your regular mongd.

Upvotes: 2

Sammaye
Sammaye

Reputation: 43884

MongoDb will actually pre-alloc 2-3GB of journal upon starting mongod, this as @balafi states is the default behaviour and you must explicitly tell MongoDB not to do this, via --smallfiles ( http://docs.mongodb.org/manual/reference/mongod/#cmdoption-mongod--smallfiles ) and/or --noprealloc ( http://docs.mongodb.org/manual/reference/mongod/#cmdoption-mongod--noprealloc ).

It is normally OK to suffer a bit of pre-allocation lag unless you are in the middle of a production shutdown and every minute = money^2.

So I would recommend checking how you are starting mongod and see if you have some options which are forcing it to start:

  • without journal
  • with smallfiles
  • with no prealloc

Upvotes: 1

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