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
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 datanodestorages 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.