Re: [Sequoiadb-discuss] Sequoia

[Emmanuel C. <ma...@fr...>]

Hi Gary,

I am CC-ing the new mailing list.
> Recently saw your post on Sequoia mailing list and have a quick newbie 
> question. Does Sequoia handle write requests? How exactly does Sequoia 
> scales with write requests? I understand that for read requests, it 
> would scale somewhat (hopefully) linearly. That is, I expect that when 
> I add 2x more databases to the group, it should be able to handle 2x 
> more incoming requests. But what about write requests? Are the data 
> updates replicated by Sequoia? If so, what kind of performance and 
> delay can I expect? Any help or hint is much appreciated!
If you are using RAIDb-1, writes are sent in parallel to all databases. 
If you have a single controller, the overhead is about a millisecond in 
latency per query.
If you have multiple controllers, you have to add the group 
communication latency which varies depending on your configuration. 
Typical numbers range from 1 to 8ms of added latency to a request.
So if you have a single threaded application that perform writes, you 
will see a significant overhead especially if your writes are small 
(adding 1 ms to a sub-millisecond query is more than doubling its 
execution time). As writes also have to be executed in the same order by 
all databases, Sequoia adds some constraints to make sure that writes to 
the same table are done sequentially. If you were updating in parallel 
the same table, this will now become sequential. Therefore you can see a 
significant slowdown in such use case too (we are talking pure writes, 
no reads here).
As you pointed it out, reads are distributed among the backends and you 
can achieve fairly easily a linear speedup. So if you inject twice the 
load, the response time will remain constant and your throughput 
doubles. But if your number of requests remain constant, adding nodes to 
a non-overloaded system is not likely to bring any noticeable 
performance improvement.

I hope this clarifies things a bit.
Thanks for your interest in Sequoia,
Emmanuel

-- 
Emmanuel Cecchet
FTO @ Frog Thinker 
Open Source Development & Consulting
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Web: http://www.frogthinker.org
email: ma...@fr...
Skype: emmanuel_cecchet