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https://issues.apache.org/jira/browse/HDFS-14963?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16969795#comment-16969795
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Xudong Cao commented on HDFS-14963:
-----------------------------------
cc [~shv] [~elgoiri] [~vagarychen] [~weichiu] Thank you all for your attention.
For the convenience of reading, I have uploaded an additional patch besides
github PR (they are exactly a same patch). Based on this patch:
# The cache directory is configurable by a newly introduced item
"dfs.client.failover.cache-active.dir", its default value is
${java.io.tmpdir}, which is /tmp on Linux platform.
# Writing/Reading a cache file is under file lock protection, and we use
trylock() instead of lock(), so in a high-concurrency scenario, reading/writing
cache file will not become the bottleneck. if trylock() failed while reading,
it just fall back to what we have today: begin with 1st NN. And if trylock()
failed while writing, it simply returns and continues. In fact, I think both
these situations should be very rare.
# All cache files' mode are manually set to "666", meaning every process can
read/write them.
# This cache mechanism is robust, regardless of whether the cache file was
accidentally deleted or the content was maliciously modified, the
readActiveCache() always returns a legal index, and writeActiveCache() will
automatically rebuild the cache file on next failover.
# We surely have dfs.client.failover.random.order, actually I have used it in
the unit test, Zkfc does know who is active NN right now, but it does not have
an rpc interface allowing us to get it. and I think an rpc call is much more
expensive than reading/writing local files.
# cc [~elgoiri], I will then tacle the logging issue discussed in (2) in a
separate JIRA.
> Add HDFS Client machine caching active namenode index mechanism.
> ----------------------------------------------------------------
>
> Key: HDFS-14963
> URL: https://issues.apache.org/jira/browse/HDFS-14963
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: hdfs-client
> Affects Versions: 3.1.3
> Reporter: Xudong Cao
> Assignee: Xudong Cao
> Priority: Minor
> Attachments: HDFS-14963.000.patch
>
>
> In multi-NameNodes scenery, a new hdfs client always begins a rpc call from
> the 1st namenode, simply polls, and finally determines the current Active
> namenode.
> This brings at least two problems：
> # Extra failover consumption, especially in the case of frequent creation of
> clients.
> # Unnecessary log printing, suppose there are 3 NNs and the 3rd is ANN, and
> then a client starts rpc with the 1st NN, it will be silent when failover
> from the 1st NN to the 2nd NN, but when failover from the 2nd NN to the 3rd
> NN, it prints some unnecessary logs, in some scenarios, these logs will be
> very numerous:
> {code:java}
> 2019-11-07 11:35:41,577 INFO retry.RetryInvocationHandler:
> org.apache.hadoop.ipc.RemoteException(org.apache.hadoop.ipc.StandbyException):
> Operation category READ is not supported in state standby. Visit
> https://s.apache.org/sbnn-error
> at
> org.apache.hadoop.hdfs.server.namenode.ha.StandbyState.checkOperation(StandbyState.java:98)
> at
> org.apache.hadoop.hdfs.server.namenode.NameNode$NameNodeHAContext.checkOperation(NameNode.java:2052)
> at
> org.apache.hadoop.hdfs.server.namenode.FSNamesystem.checkOperation(FSNamesystem.java:1459)
> ...{code}
> We can introduce a solution for this problem: in client machine, for every
> hdfs cluster, caching its current Active NameNode index in a separate cache
> file named by its uri. *Note these cache files are shared by all hdfs client
> processes on this machine*.
> For example, suppose there are hdfs://ns1 and hdfs://ns2, and the client
> machine cache file directory is /tmp, then:
> # the ns1 cluster related cache file is /tmp/ns1
> # the ns2 cluster related cache file is /tmp/ns2
> And then:
> # When a client starts, it reads the current Active NameNode index from the
> corresponding cache file based on the target hdfs uri, and then directly make
> an rpc call toward the right ANN.
> # After each time client failovers, it need to write the latest Active
> NameNode index to the corresponding cache file based on the target hdfs uri.
>
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