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https://issues.apache.org/jira/browse/HDFS-14313?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16869322#comment-16869322
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Lisheng Sun edited comment on HDFS-14313 at 6/21/19 9:24 AM:
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[~jojochuang] Thans for your review.
{{{quote}}}
{{Additionally, what's the memory consumption look like? I assume it doubles
DataNode memory usage.}}
{{{quote}}}{{}}
{{It cost memory far less than twice DataNode memory usage. }}
{code:java}
Collection<ReplicaInfo> replicaInfos =
(Collection<ReplicaInfo>) fsDataset.deepCopyReplica(bpid);
{code}
{{Because }}ReplicaCachingGetSpaceUsed#replicaInfos is local variable and used
for every block pool， and it also increases reference count.
Increased memory consumption is about 4 bytes（8 bytes） * ReplicaInfos.
{quote}
synchronization: I find it hard to believe that FsDatasetImpl#deepCopyReplica()
is not synchronized to avoid data race.
{quote}
I can't fully understand this synchronization problem. You worry about data
race happen?
was (Author: leosun08):
[~jojochuang] Thans for your review.
{{{quote}}}
{{Additionally, what's the memory consumption look like? I assume it doubles
DataNode memory usage.}}
{{{quote}}}{{}}
{{It cost memory far less than twice DataNode memory usage. }}
{code:java}
Collection<ReplicaInfo> replicaInfos =
(Collection<ReplicaInfo>) fsDataset.deepCopyReplica(bpid);
{code}
{{Because }}ReplicaCachingGetSpaceUsed#replicaInfos is local variable and used
for every block pool， and it also increases reference count.
Increased memory consumption is about 4 bytes（8 bytes） * ReplicaInfos.
{quote}
synchronization: I find it hard to believe that FsDatasetImpl#deepCopyReplica()
is not synchronized to avoid data race.
{quote}
I can't fully understand this synchronization problem. You worry about data
race happen?
> Get hdfs used space from FsDatasetImpl#volumeMap#ReplicaInfo in memory
> instead of df/du
> ----------------------------------------------------------------------------------------
>
> Key: HDFS-14313
> URL: https://issues.apache.org/jira/browse/HDFS-14313
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: datanode, performance
> Affects Versions: 2.6.0, 2.7.0, 2.8.0, 2.9.0, 3.0.0, 3.1.0
> Reporter: Lisheng Sun
> Assignee: Lisheng Sun
> Priority: Major
> Attachments: HDFS-14313.000.patch, HDFS-14313.001.patch
>
>
> There are two ways of DU/DF getting used space that are insufficient.
> # Running DU across lots of disks is very expensive and running all of the
> processes at the same time creates a noticeable IO spike.
> # Running DF is inaccurate when the disk sharing by multiple datanode or
> other servers.
> Getting hdfs used space from FsDatasetImpl#volumeMap#ReplicaInfos in memory
> is very small and accurate.
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