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https://issues.apache.org/jira/browse/HDFS-9381?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15001798#comment-15001798
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Uma Maheswara Rao G commented on HDFS-9381:
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{quote}
These internal blocks can be reported at different time. When one internal
block is reported, we still need to wait others to be reported instead of
forcing timeout.
{quote}
After pending replication timeout, it will not keep any reference to that
block. one block entry will be added back to neededReplications. While
processing neededReplications block, it will find all the current blockGroup
details. So, it should cover even recent block missed stuff right.
{quote}
But I think they are rare? We have a block of this kind when we short of DNs,
we can't choose enough DNs to schedule recovery at once, so we shedule twice.
{quote}
Point here is one block is enough to happen this case. When neededReplications
just sent for replication and another block comes immediately, then if you just
skip with out removing from neededReplications, the same block will be picked
again and again and return back. Since NN fsnLock is very important for other
operations in general. My point was why this block processing should do
something under lock for no purpose? Yeah overall case can be rare in good
condition less failures cluster :-)
> When same block came for replication for Striped mode, we can move that block
> to PendingReplications
> ----------------------------------------------------------------------------------------------------
>
> Key: HDFS-9381
> URL: https://issues.apache.org/jira/browse/HDFS-9381
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Components: erasure-coding, namenode
> Affects Versions: 3.0.0
> Reporter: Uma Maheswara Rao G
> Assignee: Uma Maheswara Rao G
> Attachments: HDFS-9381.00.patch
>
>
> Currently I noticed that we are just returning null if block already exists
> in pendingReplications in replication flow for striped blocks.
> {code}
> if (block.isStriped()) {
> if (pendingNum > 0) {
> // Wait the previous recovery to finish.
> return null;
> }
> {code}
> Here if we just return null and if neededReplications contains only fewer
> blocks(basically by default if less than numliveNodes*2), then same blocks
> can be picked again from neededReplications from next loop as we are not
> removing element from neededReplications. Since this replication process need
> to take fsnamesystmem lock and do, we may spend some time unnecessarily in
> every loop.
> So my suggestion/improvement is:
> Instead of just returning null, how about incrementing pendingReplications
> for this block and remove from neededReplications? and also another point to
> consider here is, to add into pendingReplications, generally we need target
> and it is nothing but to which node we issued replication command. Later when
> after replication success and DN reported it, block will be removed from
> pendingReplications from NN addBlock.
> So since this is newly picked block from neededReplications, we would not
> have selected target yet. So which target to be passed to pendingReplications
> if we add this block? One Option I am thinking is, how about just passing
> srcNode itself as target for this special condition? So, anyway if the block
> is really missed, srcNode will not report it. So this block will not be
> removed from pending replications, so that when it is timed out, it will be
> considered for replication again and that time it will find actual target to
> replicate while processing as part of regular replication flow.
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