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From aaron morton <aa...@thelastpickle.com>
Subject Re: Replication factor
Date Thu, 24 May 2012 09:54:49 GMT
ReadRepair means including all UP replicas in the request, waiting asynchronously after the
read has completed, resolving and repairing differences. If you read at QUOURM with RR running,
ALL (replace) nodes will perform a read. 

At any CL > ONE the responses from CL nodes are reconciled and differences are repaired
using a mechanism similar to RR. This has to happen before the response can be sent to the
client.

The naming does not help, but they are a different. RR is a background process designed to
reduce the chance of an inconsistent read. It's needed less since changed to Hinted Handoff
in 1.0.

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

On 24/05/2012, at 5:47 AM, Daning Wang wrote:

> Thanks guys.
> 
> Aaron, I am confused about this. from wiki http://wiki.apache.org/cassandra/ReadRepair,
looks for any consistency level. Read Repair will be done either before or after responding
data.
> 
>   >Read Repair does not run at CL ONE
> 
> Daning
> 
> On Wed, May 23, 2012 at 3:51 AM, Viktor Jevdokimov <Viktor.Jevdokimov@adform.com>
wrote:
> > When RF == number of nodes, and you read at CL ONE you will always be reading locally.
> 
> “always be reading locally” – only if Dynamic Snitch is “off”. With dynamic
snitch “on” request may be redirected to other node, which may introduce latency spikes.
> 
>  
> 
>  
> 
> 
> 
> Best regards / Pagarbiai
> Viktor Jevdokimov
> Senior Developer
> 
> Email: Viktor.Jevdokimov@adform.com
> Phone: +370 5 212 3063, Fax +370 5 261 0453
> J. Jasinskio 16C, LT-01112 Vilnius, Lithuania
> Follow us on Twitter: @adforminsider
> What is Adform: watch this short video
> <signature-logo7789.png> 
> 
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> 
> From: aaron morton [mailto:aaron@thelastpickle.com] 
> Sent: Wednesday, May 23, 2012 13:00
> To: user@cassandra.apache.org
> Subject: Re: Replication factor
> 
>  
> 
> RF is normally adjusted to modify availability (see http://thelastpickle.com/2011/06/13/Down-For-Me/)
> 
>  
> 
> for example, if I have 4 nodes cluster in one data center, how can RF=2 vs RF=4 affect
read performance? If consistency level is ONE, looks reading does not need to go to another
hop to get data if RF=4, but it would do more work on read repair in the background.
> 
> Read Repair does not run at CL ONE.
> 
> When RF == number of nodes, and you read at CL ONE you will always be reading locally.
But with a low consistency.
> 
> If you read with QUORUM when RF == number of nodes you will still get some performance
benefit from the data being read locally.
> 
>  
> 
> Cheers
> 
>  
> 
>  
> 
> -----------------
> 
> Aaron Morton
> 
> Freelance Developer
> 
> @aaronmorton
> 
> http://www.thelastpickle.com
> 
>  
> 
> On 23/05/2012, at 9:34 AM, Daning Wang wrote:
> 
> 
> 
> 
> Hello,
> 
> What is the pros and cons to choose different number of replication factor in term of
performance? if space is not a concern.
> 
> for example, if I have 4 nodes cluster in one data center, how can RF=2 vs RF=4 affect
read performance? If consistency level is ONE, looks reading does not need to go to another
hop to get data if RF=4, but it would do more work on read repair in the background.
> 
> Can you share some insights about this?
> 
> Thanks in advance,
> 
> Daning
> 
>  
> 
> 


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