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From aaron morton <aa...@thelastpickle.com>
Subject Re: Time to wait for CF to be consistent after stopping writes.
Date Thu, 28 Oct 2010 10:31:58 GMT
Memtable Data Size is the number of bytes you Column Family is taking up. 

Have you read the section on "Repairing missing or inconsistent data" here http://wiki.apache.org/cassandra/Operations
. It describes the nodetool repair operation used to repair inconsistent data. 

Aaron

On 28 Oct 2010, at 22:35, Utku Can Topçu wrote:

> Gary, Thank you for your comments.
> 
> I also have another question in mind: 
> - If in all nodes "nodetool cfstats" shows that the memtable size is 0. Then can I be
sure that it's safe to assume that all values are consistent?
> 
> Regards,
> Utku
> 
> On Wed, Oct 27, 2010 at 3:24 PM, Gary Dusbabek <gdusbabek@gmail.com> wrote:
> On Wed, Oct 27, 2010 at 05:08, Utku Can Topçu <utku@topcu.gen.tr> wrote:
> > Hi,
> >
> > For a columnfamily in a keyspace which has RF=3, I'm issuing writes with
> > ConsistencyLevel.ONE.
> >
> > in the configuration I have:
> > - memtable_flush_after_mins : 30
> > - memtable_throughput_in_mb : 32
> >
> > I'm writing to this columnfamily continuously for about 1 hour then stop
> > writing.
> >
> > So the question is:
> >
> > How long should I wait after stopping writes to that particular CF so that
> > all writes take place and data contained in the CF will be consistent.
> 
> There is no way to determine this precisely.  Depending on your nodes
> and network it could be as short as a few milliseconds or much longer.
> 
> > Which metrics should I be checking to ensure that the CF is now consistent?
> 
> Execute a read using ConsistencyLevel.ALL.  If the value is not yet
> consistent, read repair will ensure that it soon will be.  Another
> approach is to write using ConsistencyLevel.ALL, although that would
> decrease your write throughput.
> 
> >
> > And additionally if I was using ConsistencyLevel.QUORUM or
> > ConsistencyLevel.ALL would it make a difference?
> 
> Precisely.
> 
> > Would reducing the RF=3 to RF=1 would it make my life on this decision
> > easier?
> 
> It would make determining consistency better, but RF=1 isn't going to
> be very fault tolerant.
> 
> Gary.
> 


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