Mongo 3.0 - RocksDB

[Aishwarya Ganesan]

Hi,

I would like to use mongoDB 3.0 with rocksdb as storage engine.  If my writes are with j: true, does this guarantee that writes go to the journal before it is acknowledged?

Is there a rocksdb config that I should change or j:true in my query takes care of it?

Thanks,

Aishwarya

[Igor Canadi]

Hi Aishwarya,

MongoDB with RocksDB sends all writes to the journal if you enabled the journal. This behavior does not depend on the value of `j` write concern option.

Best,

Igor

[Aishwarya Ganesan]

Thanks for the reply.  I wasn't clear in my question. I notice that the journal is getting created in my mongodb data directory and also that writes are going to the journal. 

I want to make sure that the journal is fsynced before the write is acknowledged. Is j: true sufficient to achieve this?

[Igor Canadi]

Ah, I see. I'm not sure if you actually fsync the write to the journal with `j: true` option. Rocks behavior in this point is equivalent to WiredTiger's, since we expose the same function in the storage engine API, that asks us to fsync the write to journal:

1. rocks' waitUntilDurable: https://github.com/mongodb-partners/mongo-rocks/blob/master/src/rocks_recovery_unit.cpp#L260

2. wt's waitUntilDurable: https://github.com/mongodb/mongo/blob/master/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.cpp#L148

But I'm not sure what's the WT's behavior like. In the documentation it says "j:true will acknowledge the write once it gets written to the journal", but it doesn't specify whether the commit will be fsync-ed or not.

Here's how you could verify it: strace the mongod process and send it a write with j: true. If you see 'fsync' happening, than waitUntilDurable() has been called and the write is fsynced.

Hope this answers your question.

Best,

Igor

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[Asya Kamsky]

The write will be acknowledged when the journal is synced to disk.

Asya

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