On Sun, May 8, 2011 at 8:04 PM, Simon Tian <[email protected]> wrote:
> For primary copy, I think when the replication size is 3, 4, or even
> more, the writing speed should also near with 2 replication. Because
> the 2nd, 3rd, 4th, ... replication are written parallelly. The speed
> I got for 3, 4 replication is not near with the speed of 2, in fact,
> like linear reduce.
You're hitting your network limits there. With primary copy then the
primary needs to send out the data to each of the replicas, which caps
the write speed at (network bandwidth) / (num replicas). Presumably
you're using a gigabit network (or at least your nodes have gigabit
connections):
1 replica: ~125MB/s (really a bit less due to protocol overhead)
2 replicas:~62MB/s
3 replicas: ~40MB/s
4 replicas: ~31MB/s
etc.
Of course, you can also be limited by the speed of your disks (don't
forget to take journaling into account); and your situation is further
complicated by having multiple daemons per physical node. But I
suspect you get the idea. :)
-Greg
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