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From Jonathan Ellis <jbel...@gmail.com>
Subject Re: Commit Log Magic
Date Thu, 23 May 2013 13:53:52 GMT
Sstables must be sorted by token, or we can't compact efficiently.
Since writes usually do not arrive in token order, we stage them first
in a memtable.

(cc user@)

On Thu, May 23, 2013 at 8:44 AM, Ansar Rafique <ansark6@hotmail.com> wrote:
> Hi Jonathan,
>
> I am Ansar Rafique and I asked you few questions 2 week ago about Cassandra
> Implementation. I was watching your presentation where you suggested the
> page below.
>
> http://nosql.mypopescu.com/post/27684111441/cassandra-and-solid-state-drives
>
> I have a question and I have tried to find the answer but didn't really get
> satisfactory response yet. My question is why Cassandra using Commit log for
> durability instead direct write to SSTable. Cassandra acheives high write
> throughput because it stores data first in memtable and then flush into
> disk. Sounds good but remeber Cassandra also write in commit log for
> durability. I made it sure and it's written that write to memetable and
> commit log is synchronous which means it will write first in commit log and
> wait until it complete and will start writing in memtable or vice versa.
> Writing transaction to commit log requires an I/O operation which means for
> each insert we need an I/O :( for writing data in commit log and later
> requires more I/O's to flush data again on disk. Isn't writing to commit log
> is overhead ? Isn't it better to directly write data on disk instead of
> commit log ?
>
> Remember I/O operations are expensive and reduction in I/O's mean
> improvement in performance. If we look at RDBMS, it stores data in commit
> log as well as disk. Fair enough but if we don't insert data in commit log.
> It's performance should be the same as Cassandra because it perform I/O to
> insert data on disk and Cassandra also perform's I/O to insert data on
> commit log. Is commit log is less expensive ? I didn't really understood the
> magic :) Would you like to elaborate it more ?
>
> Thank you in advance for your time. Looking to hear from you.
>
> Regards,
> Ansar Rafique
>
>
>
>



-- 
Jonathan Ellis
Project Chair, Apache Cassandra
co-founder, http://www.datastax.com
@spyced

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