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From Jonathan Colby <jonathan.co...@gmail.com>
Subject Re: simple question about merged SSTable sizes
Date Wed, 22 Jun 2011 17:00:37 GMT
Thanks for the explanation.  I'm still a bit "skeptical".   

So if you really needed to control the maximum size of compacted SSTables,  you need to delete
data at such a rate that the new files created by compaction are less than or equal to the
sum of the segments being merged.

Is anyone else running into really large compacted SSTables that gave you trouble with hard
disk capacity?  How did you deal with it?   

We have 1 TB disks in our nodes, but keeping in mind we need to have at least 50% for the
worst case compaction scenario I'm still a bit worried that one day we're going to hit a dead
end.



On Jun 22, 2011, at 6:50 PM, Eric tamme wrote:

> On Wed, Jun 22, 2011 at 12:35 PM, Jonathan Colby
> <jonathan.colby@gmail.com> wrote:
>> 
>> The way compaction works,  "x" same-sized files are merged into a new SSTable.  This
repeats itself and the SSTable get bigger and bigger.
>> 
>> So what is the upper limit??     If you are not deleting stuff fast enough, wouldn't
the SSTable sizes grow indefinitely?
>> 
>> I ask because we have some rather large SSTable files (80-100 GB) and I'm starting
to worry about future compactions.
>> 
>> Second, compacting such large files is an IO killer.    What can be tuned other than
compaction_threshold to help optimize this and prevent the files from getting too big?
>> 
>> Thanks!
> 
> 
> The compaction is an iterative process that first compacts uncompacted
> SSTables and removes tombstones etc.  This compaction takes multiple
> files and merges them into one SSTable.  This process repeats until
> you have "compaction_threshold=X" number of similarly sized SSTables,
> then those will get re-compacted (merged) together.  The number and
> size of SSTables that you have as a result of a flush is tuned by max
> size, or records, or time.  Contrary to what you might believe, having
> fewer larger SSTables reduces IO compared to compacting many small
> SSTables.  Also the merge operation of previously compacted SSTables
> is relatively fast.
> 
> As far as I know, cassandra will continue compacting SSTables into an
> indefinitely larger sized SSTable.  The tunable side of things is for
> adjusting when to flush memtable to SSTable, and the number of
> SSTables of similar size that must be present to execute a compaction.
> 
> -Eric


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