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From Vanger <disc...@gmail.com>
Subject Re: Adding node to Cassandra
Date Mon, 12 Mar 2012 11:38:49 GMT
We were aware of compaction overhead, but still don't understand why 
that shall happened: node 'D' was in stable condition, works for at 
least month, had all data for its token range and was comfortable with 
such disk space.
Why suddenly node needs 2x more space for data it already have? Why 
decreasing token range not lead to decreasing disk usage?

On 12.03.2012 15:14, Rustam Aliyev wrote:
> Hi,
>
> If you use SizeTieredCompactionStrategy, you should have x2 disk space 
> to be on the safe side. So if you want to store 2TB data, you need 
> partition size of 4TB at least.  LeveledCompactionStrategy is 
> available in 1.x and supposed to require less free disk space (but 
> comes at price of I/O).
>
> --
> Rustam.
>
> On 12/03/2012 09:23, Vanger wrote:
>> *We have cassandra 4 nodes cluster* with RF = 3 (nodes named from 'A' 
>> to 'D', initial tokens:
>> *A (25%)*: 20543402371996174596346065790779111550, *
>> B (25%)*: 63454860067234500516210522518260948578,
>> *C (25%)*: 106715317233367107622067286720208938865,
>> *D (25%)*: 150141183460469231731687303715884105728),
>> *and want to add 5th node* ('E') with initial token = 
>> 164163260474281062972548100673162157075,  then we want to rebalance 
>> A, D, E nodes such way they'll own equal percentage of data. All 
>> nodes have ~400 GB of data and around ~300GB disk free space.
>> What we did:
>> 1. 'Join' new cassandra instance (node 'E') to cluster and wait 'till 
>> it loads data for it tokens range.
>>
>> 2. Move node 'D' initial token down from 150... to 130...
>> Here we ran into a problem. When "move" started disk usage for node C 
>> grows from 400 to 750GB, we saw running compactions on node 'D' but 
>> some compactions failed with /"WARN [CompactionExecutor:580] 
>> 2012-03-11 16:57:56,036 CompactionTask.java (line 87) insufficient 
>> space to compact all requested files SSTableReader"/ after that we 
>> killed "move" process to avoid "out of disk space" error (when 5GB of 
>> free space left). After restart it frees 100GB of space and now we 
>> have total of 105GB free disk space on node 'D'. Also we noticed 
>> increased disk usage by ~150GB at node 'B' but it stops growing 
>> before we stopped "move token".
>>
>>
>> So now we have 5 nodes in cluster in status like this:
>> Node, Owns%,     Load,     Init. token
>> A:         16%       400GB        020...
>> B:         25%       520GB        063...
>> C:         25%       400GB        106...
>> D:         25%       640GB        150...
>> E:          9%         300GB        164...
>>
>> We'll add disk space for all nodes and run some cleanups, but there's 
>> still left some questions:
>>
>> What is the best next step  for us from this point?
>> What is correct procedure after all and what should we expect when 
>> adding node to cassandra cluster?
>> We expected decrease of used disk space on node 'D' 'cause we shrink 
>> token range for this node, but saw the opposite, why it happened and 
>> is it normal behavior?
>> What if we'll have 2TB of data on 2.5TB disk and we wanted to add 
>> another node and move tokens?
>> Is it possible to automate node addition to cluster and be sure we 
>> won't run out of space?
>>
>> Thank.


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