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https://issues.apache.org/jira/browse/HBASE-26304?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17425263#comment-17425263
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Bryan Beaudreault commented on HBASE-26304:
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Note: the above should drastically reduce the impact of block moves, but will
not fix our localityIndex metrics. I'm going to need to find some other way to
expose on the DFSInputStream that locality has improved, which could trigger a
recalculation of localityIndex. Alternatively, we could just do it as part of a
chore in the regionserver.
> Reflect out-of-band locality improvements in served requests
> ------------------------------------------------------------
>
> Key: HBASE-26304
> URL: https://issues.apache.org/jira/browse/HBASE-26304
> Project: HBase
> Issue Type: Sub-task
> Reporter: Bryan Beaudreault
> Assignee: Bryan Beaudreault
> Priority: Major
>
> Once the LocalityHealer has improved locality of a StoreFile (by moving
> blocks onto the correct host), the Reader's DFSInputStream and Region's
> localityIndex metric must be refreshed. Without refreshing the
> DFSInputStream, the improved locality will not improve latencies. In fact,
> the DFSInputStream may try to fetch blocks that have moved, resulting in a
> ReplicaNotFoundException. This is automatically retried, but the retry will
> increase long tail latencies relative to configured backoff strategy.
> See https://issues.apache.org/jira/browse/HDFS-16155 for an improvement in
> backoff strategy which can greatly mitigate latency impact of the missing
> block retry.
> Even with that mitigation, a StoreFile is often made up of many blocks.
> Without some sort of intervention, we will continue to hit
> ReplicaNotFoundException over time as clients naturally request data from
> moved blocks.
> In the original LocalityHealer design, I created a new
> RefreshHDFSBlockDistribution RPC on the RegionServer. This RPC accepts a list
> of region names and, for each region store, re-opens the underlying StoreFile
> if the locality has changed.
> I will submit a PR with that implementation, but I am also investigating
> other avenues. For example, I noticed
> https://issues.apache.org/jira/browse/HDFS-15119 which doesn't seem ideal but
> maybe can be improved as an automatic lower-level handling of block moves.
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