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From AJ ...@dude.podzone.net>
Subject Re: Clock skew
Date Tue, 28 Jun 2011 20:59:14 GMT
Yikes!  I just read your blog Dominic.  Now I'm worried since my app was 
going to be mostly cloud-based.  But, you didn't mention anything about 
sleeping for 'max clock variance' after making the ntp-related config 
changes (maybe you haven't had the time to blog it).

I'm curious, do you think the sleep is required even in a 
non-virtualized environment?  Is it only needed when implementing some 
kind of lock?  Does the type of lock make a difference?

Thanks!
aj (the other one)

On 6/28/2011 11:31 AM, Dominic Williams wrote:
> Hi, yes you are correct, and this is a potential problem.
>
> IMPORTANT: If you need to serialize writes from your application 
> servers, for example using distributed locking, then before releasing 
> locks you must sleep for a period equal to the maximum variance 
> between the clocks on your application server nodes.
>
> I had a problem with the clocks on my nodes which led to all kinds of 
> problems. There is a slightly out of date post, which may not 
> mentioned the above point, on my experiences here 
> http://ria101.wordpress.com/2011/02/08/cassandra-the-importance-of-system-clocks-avoiding-oom-and-how-to-escape-oom-meltdown/
>
> Hope this helps
> Dominic
>
> On 27 June 2011 23:03, A J <s5alye@gmail.com 
> <mailto:s5alye@gmail.com>> wrote:
>
>     During writes, the timestamp field in the column is the system-time of
>     that node (correct me if that is not the case and the system-time of
>     the co-ordinator is what gets applied to all the replicas).
>     During reads, the latest write wins.
>
>     What if there is a clock skew ? It could lead to a stale write
>     over-riding the actual latest write, just because the clock of that
>     node is ahead of the other node. Right ?
>
>


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