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I have set up two Mongo instances in EC2 having RAID10. I set up replica set using those two servers and found that this replica is very very slow over network. If I log in locally and issue query inside mongo console all is fine. I tested disk speed no issue here:

sudo dd if=/dev/zero of=/data/output2.img bs=8k count=100k
102400+0 records in
102400+0 records out
838860800 bytes (839 MB) copied, 1.87101 s, 448 MB/s

And similar set up that is without replication works fast. How can I track down this issue ?

share|improve this question

This is a bit light on details - you don't mention some vital information like what exactly is slow (is this access via a driver for example). Replication adds a little load to the primary, but in general is pretty lightweight.

What size instances are you running, and where (replicating across regions etc.)? You might be saturating your available bandwidth when you add the replication data to the mix. I would only expect that though if you were running a write heavy application or if perhaps you were doing slaveOK reads.

Finally, as a general guide to running MongoDB in AWS, check out this white paper:

And these notes in the official docs:

share|improve this answer
I was looking for the issue for long time now, I know its light on details. Thanks for your help ! I have to time queries on our application side to know the answer, seems that mongo is ok. – Lukasz Mar 13 '12 at 8:21
From this comment, it seems that this question is no longer an open one, maybe consider closing it? – Mike Fiedler Apr 22 '12 at 12:46
How can I close it ? Should I delete it ? – Lukasz May 30 '12 at 9:41
I think he means you just accept an answer and it will then get closed :) – Adam C May 30 '12 at 21:20

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