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https://issues.apache.org/jira/browse/HDFS-9381?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Uma Maheswara Rao G updated HDFS-9381:
--------------------------------------
Description:
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.
was:
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 neededReplications contains only fewer blocks(basically by default if
less than numliveNodes*2), then same blocks can be picked again from
neededReplications if we just return null 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 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 block is really
missed, srcNode anyway will not report it. So this block will not be removed
from pending replications, so that when it timeout, it will be considered for
replication and that time it will find actual target to replicate.
So
> 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
>
> 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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