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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]# [Openstack] [Swift] Erasure code durability and overhead in small clusters

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On 14/03/18 13:33, Mark Kirkwood wrote: > > Doing a bit more playing about leads me to think that for a *single > region* EC policy we can get a tighter lower bound on the number of > hosts: I'm calculating it as (k+m)/m. I probably should have shared the reasoning rather than just plumping the formula out! So here it is: Suppose we have n hosts. The k + m fragments get spread about these n hosts,. On average, therefore (k + m)/n will be on any one of them. We want to ensure that this number is <= m (so one losing 1 host does not take out too many fragments). i.e: (k + m)/n <= m Doing schoolboy algebra on this gets: (k + m)/m <= n Which is the stated conjecture for the minimum number of hosts.

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