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From Edward Capriolo <edlinuxg...@gmail.com>
Subject Re: Stretched HDFS cluster
Date Wed, 16 Sep 2009 18:39:30 GMT
On Wed, Sep 16, 2009 at 7:27 AM, Steve Loughran <stevel@apache.org> wrote:
> Touretsky, Gregory wrote:
>>
>> Hi,
>>
>>    Does anyone have an experience running HDFS cluster stretched over
>> high-latency WAN connections?
>> Any specific concerns/options/recommendations?
>> I'm trying to setup the HDFS cluster with the nodes located in the US,
>> Israel and India - considering it as a potential solution for cross-site
>> data sharing...
>>
>
> I would back up todd here and say "don't do it -yet". I think there are some
> minor placeholders in the rack hierarchy to have an explicit notion of
> different sites, but nobody has done the work yet. Cross datacentre data
> balancing and work scheduling is complex, and all the code in Hadoop,
> zookeeper, etc, is built on the assumption that latency is low, all machines
> clocks are going forward at roughly the same rate, the network is fairly
> reliable, routers are unlikely to corrupt data, etc.
>
> Now, if you do want to do >1 site, it would be a profound and useful
> development -I'd expect the MR scheduler, or even the Pig/Hive code
> generators , to take datacentre locality into account, doing as much work
> per site as possible. The problem of block distribution changes too, as you
> would want 1 copy of each block in the other datacentre. Even then, I'd
> start with sites in a single city, on a MAE or other link where bandwidth
> matters less. Note that (as discussed below) on the MAN scale things can
> start to go wrong in ways that are unlikely in a datacentre, and its those
> failures that will burn you
>
> worth reading
> http://status.aws.amazon.com/s3-20080720.html
> http://www.allthingsdistributed.com/2008/12/eventually_consistent.html
>
> -Steve
>
On a somewhat related topic I was showing a co-worker a Hadoop setup
and he asked stated, "What if we got a bunch of laptops on the
internet like the playstation 'Folding @ Home'" of course these are
widely different distributed models.

I have been thinking about this. Assume:
Throw the data security out the window, and assume everyone is playing fair.
Assume we have systems with a semi-dedicated IP, like my cable
internet. with no inbound/outbound restrictions.
Assume every computer is its own RACK
LAN is very low latency Assume that latency is like 40 ms
Assume we tune up replication to say 10 or higher to deal with drop on/drop offs

Could we run a 100 Node cluster? If no what is stopping it from working?

My next question. For fun, does anyone want to try? We could setup
IPTABLES/firewall allowing hadoop traffic from IP's in the experiment.
I have two nodes in Chicago, US ISP to donate. Even if we get 10 nodes
that would be interesting as a benchmark.

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