Bryan Beaudreault created HBASE-26304:
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Summary: 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
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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