MongoDB – How to Limit Database Size

I am not very familiar with MongoDB, but from this link:

As soon as a datafile starts to be used, the next one will be preallocated.

It seems to me that (on *nix at least), MongoDB will always have a datafile that is empty, or mostly empty. If it's writing heavy and filling up the file, the last file might have some data in it before a new empty file can be created.

Additionally, on Unix, mongod will preallocate an additional datafile in the background and do background initialization of this file. These files are prefilled with zero bytes.

So to determine if it's used, check the file for non-zero bytes.

OK, we solved this.

Issue is that mongo auto creates the collection when incoming data comes in.

So the problem was:

1. Drop collection
2. // transactions are coming in
3. Create collection as capped

But #3 comes too late, as the collection is already present, as not capped, because of #2.

Solution:

• stop mongo
• bring up mango on non standard port // to prevent auto creation of collection
• create collection as capped
• stop mongo, bring it back up on regular port

Now I have a capped collection!