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From Evan Jones <ev...@MIT.EDU>
Subject Re: zookeeper on ec2
Date Mon, 06 Jul 2009 20:43:22 GMT
On Jul 6, 2009, at 15:40 , Henry Robinson wrote:
> This is an interesting way of doing things. It seems like there is a
> correctness issue: if a majority of servers fail, with the remaining
> minority lagging the leader for some reason, won't the ensemble's  
> current
> state be forever lost? This is akin to a majority of servers failing  
> and
> never recovering. ZK relies on the eventual liveness of a majority  
> of its
> servers; with EC2 it seems possible that that property might not be
> satisfied.

I think you are absolutely correct. However, my understanding of EC2  
failure modes is that even though there is no guarantee that a  
particular instance's disk will survive a failure, it is very possible  
to observe EC2 nodes that "fail" temporarily (such as rebooting). In  
these cases, the instance's disk typically does survive, and when it  
comes back it will have the same contents. It is only "permanent" EC2  
failures where the disk is gone (eg. hardware failure, or Amazon  
decides to pull it for some other reason).

Thus, this looks a lot like running your own machines in your own data  
center to me. Soft failures will recover, hardware failures won't. The  
only difference is that if you were running the machines yourself, and  
you ran into some weird issue where you had hardware failures across a  
majority of your Zookeeper ensemble, you could physically move the  
disks to recover the state. If this happens in EC2, you will have to  
do some sort of "manual" repair where you forcibly restart Zookeeper  
using the state of one of the surviving members. Some Zookeeper  
operations may be lost in this case.

However, we are talking about a situation that seems exceedingly rare.  
No matter what kind of system you are running, serious non-recoverable  
failures will happen, so I don't see this to be an impediment for  
running Zookeeper or other quorum systems in EC2.

That said, I haven't run enough EC2 instances for a long enough period  
of time to observe any serious failures or recoveries. If anyone has  
more detailed information, I would love to hear about it.

Evan Jones

--
Evan Jones
http://evanjones.ca/


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