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From Rustam Aliyev <rus...@code.az>
Subject Re: Adding node to Cassandra
Date Mon, 12 Mar 2012 14:38:10 GMT
It's hard to answer this question because there are whole bunch of 
operations which may cause disk usage growth - repair, compaction, move 
etc. Any combination of these operations will make things only worse. 
But let's assume that in your case the only operation increasing disk 
usage was "move".

Simply speaking "move" does not move data from one node to another, it 
just copies data. Once data copied, you need to cleanup data which node 
is not responsible for using "cleanup" command.

If you can't increase storage, maybe you can try moving nodes slowly. 
I.e. Instead of moving node D from 150... to 130..., try going first to 
140..., cleanup and then from 140... to 130... However, I never tried 
this and can't guarantee that it will use less disk space.

In the past, someone reported x2.5 increase when they went from 4 nodes 
to 5.

--
Rustam.

On 12/03/2012 12:46, Vanger wrote:
> Cassandra v1.0.8
> once again: 4-nodes cluster, RF = 3.
>
>
> On 12.03.2012 16:18, Rustam Aliyev wrote:
>> What version of Cassandra do you have?
>>
>> On 12/03/2012 11:38, Vanger wrote:
>>> 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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