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From Narendra Sharma <narendra.sha...@gmail.com>
Subject Re: Does the memtable replace the old version of column with the new overwriting version or is it just a simple append ?
Date Wed, 09 Mar 2011 07:05:45 GMT
Multiple write for same key and column will result in overwriting of column
in a memtable. Basically multiple updates for same (key, column) are
reconciled based on the column's timestamp. This happens per memtable. So if
a memtable is flushed to an sstable, this rule will be valid for the next
memtable.
Note that sstables are immutable. So, different sstables may have different
versions of same (key, column), and the reconciliation of that happens
during read (read repair). This is why reads are slower than writes because
conflict resolution happens during read.

Hope this answers the question!

Thanks,
-Naren

On Tue, Mar 8, 2011 at 10:44 PM, Aditya Narayan <adynnn@gmail.com> wrote:

> Do the overwrites of newly written columns(that are present in
> memtable) *replace the old column* or is it just a simple append.
>
> I am trying to understand that if I update these column very very
> frequently(while they are in memtable), does the read performance of
> these columns gets affected, since Cassandra will have to read so many
> versions of the same column. If this is just replacement with old
> column then I guess read will be much better since it needs to see
> just single existing version of column.
>
> Thanks
> Aditya Narayan
>

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