On 23 Oct 2014, at 21:29 , Robert Coli <rc...@eventbrite.com> wrote:

> On Thu, Oct 23, 2014 at 9:33 AM, Sean Bridges <sean.brid...@gmail.com> wrote:
> The change from parallel to sequential is very dramatic.  For a small cluster 
> with 3 nodes, using cassandra 2.0.10,  a parallel repair takes 2 hours, and 
> io throughput peaks at 6 mb/s.  Sequential repair takes 40 hours, with 
> average io around 27 mb/s.  Should I file a jira?
> 
> As you are an actual user actually encountering the problem I had only 
> conjectured about, you are the person best suited to file such a ticket on 
> the reasonableness of the -par default. :D

Hm?  I’ve been banging my head against the exact same problem (cluster size 
five nodes, RF=3, ~40GB/node) - paraller repair takes about 6 hrs whereas 
serial takes some 48 hours or so. In addition, the compaction impact is roughly 
the same - that is, there’s the same number of compactions triggered per 
minute, but serial runs eight times more of them. There does not seem to be a 
difference between the node response latency during parallel or serial repair.

NB: We do increase our compaction throughput during calmer times, and lower it 
through busy times, and the serial compaction takes enough time to hit the busy 
period - that might also have an impact to the overall performance.

If I had known that this had so far been a theoretical problem, I would’ve 
spoken up earlier. Perhaps serial repair is not the best default.

/Janne

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