zhengchenyu created HDFS-15715:
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Summary: ReplicatorMonitor performance degradation, when the
storagePolicy of many file are not match with their real datanodestorage
Key: HDFS-15715
URL: https://issues.apache.org/jira/browse/HDFS-15715
Project: Hadoop HDFS
Issue Type: Bug
Components: hdfs
Affects Versions: 3.2.1, 2.7.3
Reporter: zhengchenyu
Assignee: zhengchenyu
Fix For: 3.3.1
One of our Namenode which has 300M files and blocks. In common way, this namode
shoud not be in heavy load. But we found rpc process time keep high, and
decommission is very slow.
I search the metrics, I found uderreplicated blocks keep high. Then I jstack
namenode, found 'InnerNode.getLoc' is hot spot cod. I think maybe chooseTarget
can't find block, so result to performance degradation. Consider with
HDFS-10453, I guess maybe some logical trigger to the scene where chooseTarget
can't find proper block.
Then I enable some debug. (Of course I revise some code so that only debug
isGoodTarget, because enable BlockPlacementPolicy's debug log is dangrouse). I
found "the rack has too many chosen nodes" is called. Then I found some log
like this
{code}
2020-12-04 12:13:56,345 WARN
org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicy: Failed to
place enough replicas, still in need of 0 to reach 3 (unavailableStorages=[],
storagePolicy=BlockStoragePolicy{HOT:7, storageTypes=[DISK],
creationFallbacks=[], replicationFallbacks=[ARCHIVE]}, newBlock=false) For more
information, please enable DEBUG log level on
org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicy
2020-12-04 12:14:03,843 WARN
org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicy: Failed to
place enough replicas, still in need of 0 to reach 3 (unavailableStorages=[],
storagePolicy=BlockStoragePolicy{COLD:2, storageTypes=[ARCHIVE],
creationFallbacks=[], replicationFallbacks=[]}, newBlock=false) For more
information, please enable DEBUG log level on
org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicy
{code}
Then through some debug and simulation, I found the reason, and reproduction
this exception.
The reason is that some developer use COLD storage policy and mover, but the
operatiosn of setting storage policy and mover are asynchronous. So some file's
real datastorages are not match with this storagePolicy.
Let me simualte this proccess. If /tmp/a is create, then have 2 replications
are DISK. Then set storage policy to COLD. When some logical trigger(For
example decommission) to copy this block. chooseTarget then use
chooseStorageTypes to filter real needed block. Here the size of variable
requiredStorageTypes which chooseStorageTypes returned is 3. But the size of
result is 2. But 3 means need 3 ARCHIVE storage. 2 means bocks has 2 DISK
storage. Then will request to choose 3 target. choose first target is right,
but when choose seconde target, the variable 'counter' is 4 which is larger
than maxTargetPerRack which is 3 in function isGoodTarget. So skip all
datanodestorage. Then result to bad performance.
I think chooseStorageTypes need to consider the result, when the exist
replication doesn't meet storage policy's demand, we need to remove this from
result.
I changed by this way, and test in my unit-test. Then solve it.
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