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https://issues.apache.org/jira/browse/HADOOP-13055?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15733822#comment-15733822
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Andrew Wang commented on HADOOP-13055:
--------------------------------------
Thanks for working on this Zhe and Manoj. I gave the patch a look, had some
high-level comments to go over first.
It looks like this is mostly config changes to allow mounting a filesystem on
/. It doesn't enable nested mounts though, so if you mount an FS on /, that's
all you get. This doesn't seem that useful, since the point of ViewFS is to
combine multiple FS namespaces.
I interpret "linkMergeSlash" to mean having an FS mounted on /, but then allow
the VFS mount table to override that. Essentially, nested mounts, special cased
to mounting over the "/" mount.
Nested mounts are problematic though because we don't do path resolution fully
on the client-side. This surfaces for recursive delete; once we've deferred it
to a mounted filesystem, it won't pop back to VFS to get redirected to recurse
through a nested mount. It won't behave like "rm -rf". I haven't looked at the
full FS interface to see what else is out there.
My question is why "/" is called out as a special case for nested mounts. It
doesn't seem easier to implement, and the semantic issues with recursive ops
are still there. It's also different from a merge mount, in that merging merges
multiple FSs, while IIUC this is mounting an FS *over* another FS.
[~sanjay.radia] could you provide any more historical perspective on the intent
here?
HADOOP-8299 also provides a little hint from [~eli] in the description, where
he refers to the idea of a "default NN" for a mount table. This lets us ignore
the semantic differences with unix mounts, and simplifies the implementation as
well (first try to resolve with the VFS mount table, then fallback to the
default FS).
> Implement linkMergeSlash for ViewFileSystem
> -------------------------------------------
>
> Key: HADOOP-13055
> URL: https://issues.apache.org/jira/browse/HADOOP-13055
> Project: Hadoop Common
> Issue Type: New Feature
> Components: fs, viewfs
> Reporter: Zhe Zhang
> Assignee: Manoj Govindassamy
> Attachments: HADOOP-13055.00.patch, HADOOP-13055.01.patch,
> HADOOP-13055.02.patch, HADOOP-13055.03.patch, HADOOP-13055.04.patch
>
>
> In a multi-cluster environment it is sometimes useful to operate on the root
> / slash directory of an HDFS cluster. E.g., list all top level directories.
> Quoting the comment in {{ViewFs}}:
> {code}
> * A special case of the merge mount is where mount table's root is merged
> * with the root (slash) of another file system:
> * <ul>
> * <li> fs.viewfs.mounttable.default.linkMergeSlash=hdfs://nn99/
> * </ul>
> * In this cases the root of the mount table is merged with the root of
> * <b>hdfs://nn99/ </b>
> {code}
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