As far as I know Cassandra doesn't use internal queueing mechanism specific to replication. Cassandra sends the write the remote DC and after that it's upto the tcp/ip stack to deal with buffering. If requests starts to timeout Cassandra would use HH upto certain time. For longer outage you would have to run repair.
 
Also look at tcp/ip tuning parameters that are helpful with your scenario:
 
http://kaivanov.blogspot.com/2010/09/linux-tcp-tuning.html
 
Run iperf and test the latency.

On Wed, Sep 5, 2012 at 8:22 PM, Venkat Rama <venkata.s.rama@gmail.com> wrote:
Hi,

We have multi DC Cassandra ring with 2 DCs setup.   We use LOCAL_QUORUM for writes and reads.  The network we have seen between the DC is sometimes flaky lasting few minutes to few 10 of minutes.

I wanted to know what is the best way to measure/monitor either the lag or replication latency between the data centers.  Are there any metrics I can monitor to find the backlog of data that needs to be transferred?

Thanks in advance.

VR