[openstack-dev] [Swift] Erasure coding and geo replication

Mark Kirkwood mark.kirkwood at catalyst.net.nz
Mon Feb 15 09:00:49 UTC 2016

After looking at:


I have a question (that follows on from Bruno's) about using erasure 
coding with geo replication.

Now the example given to show why you could/should not use erasure 
coding with geo replication is somewhat flawed as it is immediately 
clear that you cannot set:

- num_data_frags > num_devices (or nodes) in a region

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


- num_parity_frags = num_data_frags

then having a region fail does not result in service outage.

So my real question is - it looks like it *is* possible to use erasure 
coding in geo replicated situations - however I may well be missing 
something significant, so I'd love some clarification here [1]!



[1] Reduction is disk usage and net traffic looks attractive

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