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From Jonathan Colby <jonathan.co...@gmail.com>
Subject Re: nodetool cleanup - results in more disk use?
Date Mon, 04 Apr 2011 12:20:26 GMT
hi Aaron -

The Datastax documentation brought to light the fact that over time, major compactions  will
be performed on bigger and bigger SSTables.   They actually recommend against performing too
many major compactions.  Which is why I am wary to trigger too many major compactions ...

http://www.datastax.com/docs/0.7/operations/scheduled_tasks
Performing Major Compaction¶

A major compaction process merges all SSTables for all column families in a keyspace – not
just similar sized ones, as in minor compaction. Note that this may create extremely large
SStables that result in long intervals before the next minor compaction (and a resulting increase
in CPU usage for each minor compaction).

Though a major compaction ultimately frees disk space used by accumulated SSTables, during
runtime it can temporarily double disk space usage. It is best to run major compactions, if
at all, at times of low demand on the cluster.







On Apr 4, 2011, at 1:57 PM, aaron morton wrote:

> cleanup reads each SSTable on disk and writes a new file that contains the same data
with the exception of rows that are no longer in a token range the node is a replica for.
It's not compacting the files into fewer files or purging tombstones. But it is re-writing
all the data for the CF. 
> 
> Part of the process will trigger GC if needed to free up disk space from SSTables no
longer needed.
> 
> AFAIK having fewer bigger files will not cause longer minor compactions. Compaction thresholds
are applied per bucket of files that share a similar size, there is normally more smaller
files and fewer larger files. 
> 
> Aaron
> 
> On 2 Apr 2011, at 01:45, Jonathan Colby wrote:
> 
>> I discovered that a Garbage collection cleans up the unused old SSTables.   But I
still wonder whether cleanup really does a full compaction.  This would be undesirable if
so.
>> 
>> 
>> On Apr 1, 2011, at 4:08 PM, Jonathan Colby wrote:
>> 
>>> I ran node cleanup on a node in my cluster and discovered the disk usage went
from 3.3 GB to 5.4 GB.  Why is this?
>>> 
>>> I thought cleanup just removed hinted handoff information.   I read that *during*
cleanup extra disk space will be used similar to a compaction.  But I was expecting the disk
usage to go back down when it finished.
>>> 
>>> I hope cleanup doesn't trigger a major compaction.  I'd rather not run major
compactions because it means future minor compactions will take longer and use more CPU and
disk.
>>> 
>>> 
>> 
> 


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