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https://issues.apache.org/jira/browse/HDFS-16830?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17635834#comment-17635834
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ASF GitHub Bot commented on HDFS-16830:
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zhengchenyu commented on PR #5132:
URL: https://github.com/apache/hadoop/pull/5132#issuecomment-1319904398
## 1 Background
After HDFS-13522, observer read on RBF is supported. For now, client and
router transmit state id of all nameservice, this may has two disadvantage:
* may carry lots of unnecessary data.
* msync must invokeConcurrent on all nameservice.
> Note: In my experience, the call which invokeConcurrent on all nameservice
will harm to the stability of dfsrouter. For example , in our production, after
apply HDFS-16283, avoid renewlease invoke on all nameservice, the
callqueuelength of router have keep very low.
## 2 How to transmit state id according to client's demand?
The problem is that client can't know the downstreaming nameservice.
But in PoolAlignmentContext::updateRequestState, we will know the
nameservice, we could record it in the call.
Here is an example:
* Initial state: There are no state id in client. Client make call1 without
any federated state id.
* Router will find the downstreaming nameservice, then invoke
PoolAlignmentContext::updateRequestState. Here we can record the nsid and
stateid in call1.
* When Router will return the response to client
(RouterStateIdContext::updateResponseState). we can get the nsid and stateid
from call1, then return to client.
* When client recieve the call from client, client will store the state id
of ns1.
* When client reconnect to dfsrouter, client will make call2 with ns1. This
means we transmit the state id according to client's demand.
## 3 How to msync?
In RouterClientProtocol::msync, we can get the nameservice from the current
call. Then we can invoke msync to downstreaming namenode according to client's
demand.
## 4 How to handle the first msync?
There is a problem which I did not mention in section 2. How to handle the
first mysnc?
In initial state, there are no federated state ids in client side, dfsrouter
will not invoke mysnc to any namenode.
In next call, the first call to every downstreaming namenode except msync,
RouterStateIdContext.getClientStateIdFromCurrentCall will return
Long.MIN_VALUE, will connect to active. Then the following read call to every
downstreaming namenode , will connect to observer namenode.
> Note: In fact, I prefer that do msync in router side, do not msync in
client side, just like
https://issues.apache.org/jira/secure/attachment/13011000/HDFS-13522_WIP.patch.
Then there is no problem about first msync.
## 5 What about the unit test?
/ns0 is mount on ns0, /ns1 is mount on ns1. fs0 can only transmit ns0 state
id. fs1 can only transmit ns1 state id.
Set DFS_ROUTER_OBSERVER_FEDERATED_STATE_PROPAGATION_MAXSIZE to 1, then not
allow transmit more than one state ids.
> HDFS-16830. [SBN READ] dfsrouter transmit state id according to client's
> demand
> -------------------------------------------------------------------------------
>
> Key: HDFS-16830
> URL: https://issues.apache.org/jira/browse/HDFS-16830
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: namenode, rbf
> Reporter: zhengchenyu
> Assignee: zhengchenyu
> Priority: Major
> Labels: pull-request-available
>
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