Dropping an index on 170M docs. How long will it block the DB ?

[Nicolas F]

Hi,

I have an index on two fields, with a high cardinality. I'd like to drop it, but I'm scared it would the block the db for minutes, even hours. http://docs.mongodb.org/manual/reference/command/dropIndexes/
For the record, creating this index on a new Replica Set member takes 1 hour.
I cannot block the database more than a few minutes.
The inbound traffic is 500 updates per second and I cannot stop it. I use Node.js clients, and as far as I know, they retry queries on failures. But I guess they won't retry for 2 hours :)

Thanks!

[Nicolas F]

It's in the subject, but to be clear: the collection contains 170 million documents.

[Rob Moore]

If that is the case I would use the same strategy to remove the index as is recommended for adding a new index to a replica set.

http://docs.mongodb.org/manual/tutorial/build-indexes-on-replica-sets/

Basically:

1) Stop a secondary and restart without the -replSet option and on a different port.
2) Remove the index using the shell by connecting directly to the new port.
3) Stop the member and restart as a member of the set (original port and with -replSet).

Repeat for all secondaries.

Step down the primary and repeat for the primary that is now secondary.

Rob.

[Nicolas Fouché]

Nice trick. Thanks !

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[Nicolas F]

Like you said, I dropped the index on each server after starting them out of the replicaset. It worked perfectly !
The dropIndex() command returned instantly, and nothing was written in the log. So I wonder if I could have just dropped the index on the full replicaset from the primary without any downtime.

[Asya Kamsky]

Yes, you could have dropped the index without worrying about performance.

Not sure why you thought it might take a long time - unlike building
an index, it doesn't have to do much work.

Asya

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[Nicolas Fouché]

That's what I assumed, until I saw this big red box on http://docs.mongodb.org/manual/reference/command/dropIndexes/ .

"Warning. This command obtains a write lock on the affected database and will block other operations until it has completed."

This sentence does not say it would take a long time to drop an index, but blocking the DB was a no-go for us, thus the question, to be 100% sure.

Nicolas

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