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From "sankalp kohli (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-6621) STCS fallback is not optimal when bootstrapping
Date Tue, 10 Jun 2014 22:09:02 GMT


sankalp kohli commented on CASSANDRA-6621:

"What if we just special cased LCS during bootstrap to just put streamed data into the first
level it doesn't overlap"
I agree this will be better but here is another optional improvement which can minimize stables
in L0. 

Stream stables from the source by sorting them by level which will cause streaming of stables
in following order L1 to Lx and then finally L0. Here is why this will help. 
1) If we stream an stable from higher level first, it will take a plot in L1 and will kick
other stables to higher levels or to even L0. 
2) If L0 of the streaming node is backed up and has 20-30 stables, it might end up in filling
X levels and will kick other stables to L0 due to overlapping. Streaming L0 in the end will
help in this case. 

Also I find it cleaner just to visualize that Level Z stables will go in Level Z on the node
being bootstrapped.
""where X is calculated from total dataset being streamed"
Also I am not sure whether doing the sort based improvement which I am proposing will result
in limited number of levels in the bootstrapping node. If node is bootstrapping from node
A and B and A has 5 levels and B has 3 levels. The bootstrap node will have 5 levels. So we
might not need to calculate X() 

> STCS fallback is not optimal when bootstrapping
> -----------------------------------------------
>                 Key: CASSANDRA-6621
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Bartłomiej Romański
>            Priority: Minor
> The initial discussion started in (closed) CASSANDRA-5371. I've rewritten my last comment
> After streaming (e.g. during boostrap) Cassandra places all sstables at L0. At the end
of the process we end up with huge number of sstables at the lowest level. 
> Currently, Cassandra falls back to STCS until the number of sstables at L0 reaches the
reasonable level (32 or something).
> I'm not sure if falling back to STCS is the best way to handle this particular situation.
I've read the comment in the code and I'm aware why it is a good thing to do if we have to
many sstables at L0 as a result of too many random inserts. We have a lot of sstables, each
of them covers the whole ring, there's simply no better option.
> However, after the bootstrap situation looks a bit different. The loaded sstables already
have very small ranges! We just have to tidy up a bit and everything should be OK. STCS ignores
that completely and after a while we have a bit less sstables but each of them covers the
whole ring instead of just a small part. I believe that in that case letting LCS do the job
is a better option that allowing STCS mix everything up before.
> Is there a way to disable STCS fallback? I'd like to test that scenario in practice during
our next bootstrap...
> Does Cassandra really have to put streamed sstables at L0? The only thing we have to
assure is that sstables at any given level do not overlap. If we stream different regions
from different nodes how can we get any overlaps?

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