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Scott Carey commented on HDFS1094:

This needs to change "p" from a constant, to a function of the TTR window.
"probablility of a single node failing" alone is meaningless, its concurrent failure that
is the issue. The odds of concurrent node failure is linearly proportional to TTR. I think
this model needs to assume one failure at odds = 1.0, then use the odds of concurrent failure
for the next 2 failures within the time window. A 'constant' chance of failure begs the question,
".001 chance of failure per _what_?" The first failure happens, that is assumed. Then the
next two happen given odds within a time window.
Assuming hadoop failure replication is optimized (which it isn't, the DN dishes out block
replication requests too slow).
TTR is inversely proportional to the number of racks in a group for rack failure.
TTR is inversely proportional to the number or racks in a group for single node failure _IF_
the combined bandwidth of the machines in the group in a rack is at least 2x than the betweenrack
bandwidth, otherwise it is inversely proportional to the ratio of rack bandwidth to node group
bandwidth.
The result is that only the "medium" sized groups above are viable, else it takes too long
to get data replicated when a failure happens. Also, the TTR affects the odds of data loss
on larger replication counts disproporionatly.
> Intelligent block placement policy to decrease probability of block loss
> 
>
> Key: HDFS1094
> URL: https://issues.apache.org/jira/browse/HDFS1094
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: namenode
> Reporter: dhruba borthakur
> Assignee: Rodrigo Schmidt
> Attachments: calculate_probs.py, failure_rate.py, prob.pdf, prob.pdf
>
>
> The current HDFS implementation specifies that the first replica is local and the other
two replicas are on any two random nodes on a random remote rack. This means that if any three
datanodes die together, then there is a nontrivial probability of losing at least one block
in the cluster. This JIRA is to discuss if there is a better algorithm that can lower probability
of losing a block.

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