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On 15/02/16 23:29, Kota TSUYUZAKI wrote:
> Hello Mark,
>
> AFAIK, a few reasons for that we still are in working progress for erasure code + geo replication.
>
>>> and expect to survive a region outage...
>>>
>>> With that I mind I did some experiments (Liberty swift) and it looks to me like if you have:
>>>
>>> - num_data_frags < num_nodes in (smallest) region
>>>
>>> and:
>>>
>>> - num_parity_frags = num_data_frags
>>>
>>>
>>> then having a region fail does not result in service outage.
>
> Good point but note that the PyECLib v1.0.7 (pinned to Kilo/Liberty stable) still have a problem which cannot decode the original data when all feed fragments are parity frags[1]. (i.e. if set
> num_parity_frags = num_data frags and then, num_parity_frags comes into proxy for GET request, it will fail at the decoding) The problem was already resolved in the PyECLib/liberasurecode at master
> branch and current swift master has the PyECLib>=1.0.7 dependencies so if you thought to use the newest Swift, it might be not
> a matter.


Ah right, in my testing I always took down my "1st" region...which will 
have had data fragments therein. For interest I'll try to provoke a 
situation where I have all parity ones to assemble (and see what happens).

>
> In the Swift perspective, I think that we need more tests/discussion for geo replication around write/read affinity[2] which is geo replication stuff in Swift itself and performances.
>
> For the write/read affinity, actually we didn't consider the affinity control to simplify the implementation until EC landed into Swift master[3] so I think it's time to make sure how we can use the
> affinity control with EC but it's not done yet.
>
> For the performance perspective, in my experiments, more parities causes quite performance degradation[4]. To prevent the degradation, I am working for the spec which makes duplicated copy from
> data/parity fragments and spread them out into geo regions.
>
> To sumurize, we've not done the work yet but we welcome to discuss and contribute for EC + geo replication anytime, IMO.
>
> Thanks,
> Kota
>
> 1: https://bitbucket.org/tsg-/liberasurecode/commits/a01b1818c874a65d1d1fb8f11ea441e9d3e18771
> 2: http://docs.openstack.org/developer/swift/admin_guide.html#geographically-distributed-clusters
> 3: http://docs.openstack.org/developer/swift/overview_erasure_code.html#region-support
> 4: https://specs.openstack.org/openstack/swift-specs/specs/in_progress/global_ec_cluster.html
>
>
>

Excellent - thank you for a very comprehensive answer.

Regards

Mark