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https://issues.apache.org/jira/browse/HBASE-6752?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13452486#comment-13452486
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nkeywal commented on HBASE-6752:
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bq. Who would return this? Not the server that just failed?
If we reassign immediately, the client will go to the new regionserver. So the
region server will be able to tell it a real status (for example, on reads, we
can estimate the recovery time left and the regionserver can say: come back in
20 seconds for this region).
bq. What does this mean? What kinda of priority?
Today, the split is performed by any available RS. If we preassign the regions,
the split can be done by the regionserver which is owning some of the data
we're expecting to find in the hlog file...
> On region server failure, serve writes and timeranged reads during the log
> split
> --------------------------------------------------------------------------------
>
> Key: HBASE-6752
> URL: https://issues.apache.org/jira/browse/HBASE-6752
> Project: HBase
> Issue Type: Improvement
> Components: regionserver
> Affects Versions: 0.96.0
> Reporter: nkeywal
> Priority: Minor
>
> Opening for write on failure would mean:
> - Assign the region to a new regionserver. It marks the region as recovering
> -- specific exception returned to the client when we cannot server.
> -- allow them to know where they stand. The exception can include some time
> information (failure stated on: ...)
> -- allow them to go immediately on the right regionserver, instead of
> retrying or calling the region holding meta to get the new address
> => save network calls, lower the load on meta.
> - Do the split as today. Priority is given to region server holding the new
> regions
> -- help to share the load balancing code: the split is done by region
> server considered as available for new regions
> -- help locality (the recovered edits are available on the region server)
> => lower the network usage
> - When the split is finished, we're done as of today
> - while the split is progressing, the region server can
> -- serve writes
> --- that's useful for all application that need to write but not read
> immediately:
> --- whatever logs events to analyze them later
> --- opentsdb is a perfect example.
> -- serve reads if they have a compatible time range. For heavily used
> tables, it could be an help, because:
> --- we can expect to have a few minutes of data only (as it's loaded)
> --- the heaviest queries, often accepts a few -or more- minutes delay.
> Some "What if":
> 1) the split fails
> => Retry until it works. As today. Just that we serves writes. We need to
> know (as today) that the region has not recovered if we fail again.
> 2) the regionserver fails during the split
> => As 1 and as of today/
> 3) the regionserver fails after the split but before the state change to
> fully available.
> => New assign. More logs to split (the ones already dones and the new ones).
> 4) the assignment fails
> => Retry until it works. As today.
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