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https://issues.apache.org/jira/browse/HDFS-7758?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14351058#comment-14351058
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Colin Patrick McCabe commented on HDFS-7758:
--------------------------------------------
{code}
/**
* Returns a list of volume references.
*
* The caller must release the reference of each volume by calling
* {@link FsVolumeReference#close}.
*/
public List<FsVolumeReference> getVolumeRefs();
/** Returns a reference of a given volume, specified by the index. */
public FsVolumeReference getVolumeRef(int idx) throws IOException;
{code}
This is still the wrong interface. {{getVolumeRef(int)}} encourages people to
assume that the number of volumes is never going to change. What happens if it
does?
Instead of doing this, let's have an {{Iterator}} that we can use. Something
like this:
{code}
public Iterator<FsVolumeRef> getVolumeRefIterator();
private static class FsVolumeRefIterator
implements Iterator<FsVolumeRef>, Closeable {
private final List<FsVolumeRef> list;
private int idx = 0;
FsVolumeRefIterator(List<FsVolumeSpi> spiList) {
this.list = new ArrayList<FsVolumeRef>();
for (FsVolumeSpi volume : spiList) {
try {
this.list.add(volume.obtainReference());
} catch (ClosedChannelException e) {
LOG.info("Can't obtain a reference to {} because it is closed.",
volume.getBasePath());
}
}
}
@Override
public boolean hasNext() {
return (idx < list.size());
}
@Override
public FsVolumeRef next() {
int i = idx++;
return list.get(i);
}
@Override
public void remove() {
throw UnsupportedOperationException();
}
@Override
public void close() throws IOException {
for (FsVolumeRef ref : list) {
ref.close();
}
list.clear();
}
}
{code}
Then we can get rid of {{getVolumeRefs}} and {{getVolumeRef}}. Since the
{{Iterator}} implements {{java.io.Closeable}}, findbugs will remind us that we
need to close it (and free the refs) in any function we use it in.
> Retire FsDatasetSpi#getVolumes() and use FsDatasetSpi#getVolumeRefs() instead
> -----------------------------------------------------------------------------
>
> Key: HDFS-7758
> URL: https://issues.apache.org/jira/browse/HDFS-7758
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: datanode
> Affects Versions: 2.6.0
> Reporter: Lei (Eddy) Xu
> Assignee: Lei (Eddy) Xu
> Attachments: HDFS-7758.000.patch, HDFS-7758.001.patch
>
>
> HDFS-7496 introduced reference-counting the volume instances being used to
> prevent race condition when hot swapping a volume.
> However, {{FsDatasetSpi#getVolumes()}} can still leak the volume instance
> without increasing its reference count. In this JIRA, we retire the
> {{FsDatasetSpi#getVolumes()}} and propose {{FsDatasetSpi#getVolumeRefs()}}
> and etc. method to access {{FsVolume}}. Thus it makes sure that the consumer
> of {{FsVolume}} always has correct reference count.
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