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https://issues.apache.org/jira/browse/HDFS-4257?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Harsh J updated HDFS-4257:
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Description:
Similar question has previously come over HDFS-3091 and friends, but the
essential problem is: "Why can't I write to my cluster of 3 nodes, when I just
have 1 node available at a point in time.".
The policies cover the 4 options, with {{Default}} being default:
{{Disable}} -> Disables the whole replacement concept by throwing out an error
(at the server) or acts as {{Never}} at the client.
{{Never}} -> Never replaces a DN upon pipeline failures (not too desirable in
many cases).
{{Default}} -> Replace based on a few conditions, but whose minimum never
touches 1. We always fail if only one DN remains and none others can be added.
{{Always}} -> Replace no matter what. Fail if can't replace.
Would it not make sense to have an option similar to Always/Default, where
despite _trying_, if it isn't possible to have > 1 DN in the pipeline, do not
fail. I think that is what the former write behavior was, and what fit with the
minimum replication factor allowed value.
Why is it grossly wrong to pass a write from a client for a block with just 1
remaining replica in the pipeline (the minimum of 1 grows with the replication
factor demanded from the write), when replication is taken care of immediately
afterwards? How often have we seen missing blocks arise out of allowing this +
facing a big rack(s) failure or so?
was:
Similar question has previously come over HDFS-3091 and friends, but the
essential problem is: "Why can't I write to my cluster of 3 nodes, when I just
have 1 node available at a point in time.".
The policies cover the 4 options, with {{Default}} being default:
{{Disable}} -> Disables the whole replacement concept by throwing out an error.
{{Never}} -> Never replaces a DN upon pipeline failures (not too desirable in
many cases).
{{Default}} -> Replace based on a few conditions, but whose minimum never
touches 1. We always fail if only one DN remains and none others can be added.
{{Always}} -> Replace no matter what. Fail if can't replace.
Would it not make sense to have an option similar to Always/Default, where
despite _trying_, if it isn't possible to have > 1 DN in the pipeline, do not
fail. I think that is what the former write behavior was, and what fit with the
minimum replication factor allowed value.
Why is it grossly wrong to pass a write from a client for a block with just 1
remaining replica in the pipeline (the minimum of 1 grows with the replication
factor demanded from the write), when replication is taken care of immediately
afterwards? How often have we seen missing blocks arise out of allowing this +
facing a big rack(s) failure or so?
> The ReplaceDatanodeOnFailure policies could have a forgiving option
> -------------------------------------------------------------------
>
> Key: HDFS-4257
> URL: https://issues.apache.org/jira/browse/HDFS-4257
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: hdfs-client
> Affects Versions: 2.0.2-alpha
> Reporter: Harsh J
> Priority: Minor
>
> Similar question has previously come over HDFS-3091 and friends, but the
> essential problem is: "Why can't I write to my cluster of 3 nodes, when I
> just have 1 node available at a point in time.".
> The policies cover the 4 options, with {{Default}} being default:
> {{Disable}} -> Disables the whole replacement concept by throwing out an
> error (at the server) or acts as {{Never}} at the client.
> {{Never}} -> Never replaces a DN upon pipeline failures (not too desirable in
> many cases).
> {{Default}} -> Replace based on a few conditions, but whose minimum never
> touches 1. We always fail if only one DN remains and none others can be added.
> {{Always}} -> Replace no matter what. Fail if can't replace.
> Would it not make sense to have an option similar to Always/Default, where
> despite _trying_, if it isn't possible to have > 1 DN in the pipeline, do not
> fail. I think that is what the former write behavior was, and what fit with
> the minimum replication factor allowed value.
> Why is it grossly wrong to pass a write from a client for a block with just 1
> remaining replica in the pipeline (the minimum of 1 grows with the
> replication factor demanded from the write), when replication is taken care
> of immediately afterwards? How often have we seen missing blocks arise out of
> allowing this + facing a big rack(s) failure or so?
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