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From aaron morton <aa...@thelastpickle.com>
Subject Re: Monitoring replication lag/latency in multi DC setup
Date Thu, 06 Sep 2012 23:47:59 GMT
> Is there a specific metric you can recommend?


the not entirely correct but very lightweight approach would be to look at the size of the
HintsColumnFamily in the system KS. 

If you want an exact number use the functions on the HH MBean https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/db/HintedHandOffManagerMBean.java

Note that counting the number of hits involves counting the number of hits, so that can take
a while. 

Cheers


-----------------
Aaron Morton
Freelance Developer
@aaronmorton
http://www.thelastpickle.com

On 6/09/2012, at 5:33 PM, Venkat Rama <venkata.s.rama@gmail.com> wrote:

> Is there a specific metric you can recommend?
> 
> VR
> 
> On Wed, Sep 5, 2012 at 9:19 PM, Mohit Anchlia <mohitanchlia@gmail.com> wrote:
> Cassandra exposes lot of metrics through Jconsole. You might be able to get some information
from Jconsole.
> 
> 
> On Wed, Sep 5, 2012 at 8:47 PM, Venkat Rama <venkata.s.rama@gmail.com> wrote:
> Thanks for the quick reply, Mohit.    Can we measure/monitor the size of Hinted Handoffs?
 Would it be a good enough indicator of my back log?  
> 
> Although we know when a network is flaky, we are interested in knowing how much data
is piling up in local DC that needs to be transferred.  
> 
> Greatly appreciate your help.
> 
> VR
> 
> 
> On Wed, Sep 5, 2012 at 8:33 PM, Mohit Anchlia <mohitanchlia@gmail.com> wrote:
> 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
> 
> 
> 
> 


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