I hope I get this right :) 
Thanks for contributing :)

a repair will trigger a mayor compaction on your node which will take up a lot of CPU and IO performance. It needs to do this to build up the data structure that is used for the repair. After the compaction this is streamed to the different nodes in order to repair them. 
It does not trigger a major compaction, thatís what we call running compaction on the command line and compacting all SSTables into one big one. 

it will flush all the data to disk that will create some additional compaction. 

The major concern is that s a disk IO intensive operation, it reads all the data and writes data to new SSTables (a one to one mapping). If you have all nodes doing this at the same time there may be some degraded performance. And as itís all nodes itís not possible for the Dynamic Snitch to avoid nodes if they are overloaded.

Cleanup is less intensive than repair, but itís still a good idea to stagger it. If you need to run it on all machines (or you have very powerful machines) itís probably going to be OK. 
 
Hope that helps. 

-----------------
Aaron Morton
New Zealand
@aaronmorton

Co-Founder & Principal Consultant
Apache Cassandra Consulting
http://www.thelastpickle.com

On 26/11/2013, at 5:14 am, Artur Kronenberg <artur.kronenberg@openmarket.com> wrote:

Hi Julien,

I hope I get this right :)

a repair will trigger a mayor compaction on your node which will take up a lot of CPU and IO performance. It needs to do this to build up the data structure that is used for the repair. After the compaction this is streamed to the different nodes in order to repair them.

If you trigger this on every node simultaneously you basically take the performance away from your cluster. I would expect cassandra still to function, just way slower then before. Triggering it node after node will leave your cluster with more resources to handle incoming requests.


Cheers,

Artur
On 25/11/13 15:12, Julien Campan wrote:
Hi,

I'm working with Cassandra 1.2.2 and I have a question about nodetool cleanup.
In the documentation , it's writted " Wait for cleanup to complete on one node before doing the next"

I would like to know, why we can't perform a lot of cleanup in a same time ?


Thanks