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From Vivek Mishra <vivek.mis...@impetus.co.in>
Subject please unsubscribe
Date Fri, 23 Sep 2011 09:40:43 GMT


From: Sam Overton [mailto:soverton@acunu.com]
Sent: Friday, September 23, 2011 3:09 PM
To: user@cassandra.apache.org
Subject: Re: LevelDB type compaction


On 17 September 2011 00:58, mcasandra <mohitanchlia@gmail.com<mailto:mohitanchlia@gmail.com>>
wrote:
>
>> and updates could be scattered all over
>> before compaction?
>
> No, updates to a given row will be still be in a single sstable.
Can you please explain little more? You mean that if Level 1 file contains
range from 1-100 all the updates would still go in that file?

No, sstables are never written to. All updates go to a memtable, which is then flushed to
a Level0 sstable. Compaction then causes promotion of updates to gradually higher levels.

The link on leveldb says:

> The compaction picks a file from level L and all overlapping files from
> the next level L+1

If all updates go in the same sstables then how do overlapping files get
generated. By overlapping I am assuming it means new or updated value for a
given key exists in multiple files?

Overlapping refers to the range of keys. The key ranges contained within the files in any
given level represent a set of disjoint intervals (when level >=1). For Level0, the files
may contain overlapping ranges.

Thanks for the explanation

Disclaimer: I haven't read the code, this is just my understanding from reading the docs.


Sam Overton
Acunu | http://www.acunu.com<http://www.acunu.com/> | @acunu

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