JohnZZGithub commented on a change in pull request #2185:
URL: https://github.com/apache/hadoop/pull/2185#discussion_r471831712
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File path: hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/ViewFs.md
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@@ -366,6 +366,82 @@ Don't want to change scheme or difficult to copy
mount-table configurations to a
Please refer to the [View File System Overload Scheme
Guide](./ViewFsOverloadScheme.html)
+Regex Pattern Based Mount Points
+--------------------------------
+
+The view file system mount points were a Key-Value based mapping system. It is
not friendly for user cases which mapping config could be abstracted to rules.
E.g. Users want to provide a GCS bucket per user and there might be thousands
of users in total. The old key-value based approach won't work well for several
reasons:
+
+1. The mount table is used by FileSystem clients. There's a cost to spread the
config to all clients and we should avoid it if possible. The [View File System
Overload Scheme Guide](./ViewFsOverloadScheme.html) could help the distribution
by central mount table management. But the mount table still have to be updated
on every change. The change could be greatly avoided if provide a rule-based
mount table..
+
+2. The client have to understand all the KVs in the mount table. This is not
ideal when the mountable grows to thousands of items. E.g. thousands of file
systems might be initialized even users only need one. And the config itself
will become bloated at scale.
+
+### Understand the Difference
+
+In the key-value based mount table, view file system treats every mount point
as a partition. There's several file system APIs which will lead to operation
on all partitions. E.g. there's an HDFS cluster with multiple mount. Users want
to run “hadoop fs -put file viewfs://hdfs.namenode.apache.org/tmp/” cmd to copy
data from local disk to our HDFS cluster. The cmd will trigger ViewFileSystem
to call setVerifyChecksum() method which will initialize the file system for
every mount point.
+For a regex-base rule mount table entry, we couldn't know what's corresponding
path until parsing. So the regex based mount table entry will be ignored on
such cases and the file system will be created upon accessing. The inner cache
of ViewFs is also not available for regex-base mount points now as it assumes
target file system doesn't change after viewfs initialization. Please disable
it if you want to use regex-base mount table. We also need to change the rename
strategy to SAME_FILESYSTEM_ACROSS_MOUNTPOINT for the same reason.
+```xml
+<property>
+ <name>fs.viewfs.enable.inner.cache</name>
+ <value>false</value>
Review comment:
Good call.
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