[jira] Commented: (HADOOP-6356) Add a Cache for AbstractFileSystem in the new FileContext/AbstractFileSystem framework.

["Sanjay Radia (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-6356?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12918313#action_12918313 ] 

Sanjay Radia commented on HADOOP-6356:
--------------------------------------

Dhruba says > I think it is appropriate to do the compatible-version-check for each RPC instead of doing it per creation-of-FileSystem object.

Having a FileSystem cache => RPCProxy is also cached => version check is done once per file system object. Dhruba is right in that this is incorrect since the NN may reboot with a different version.

However we will see a more serious performance problem with FileContext under certain usage patterns. FileContext keeps a pointer to the default file system; hence all file operations to the default filesystem will not require a version check (but it is susceptible to the NN reboot with different verison problem). However operations to other file systems such as fc.open("hdfs://nn/foo/bar)  will create a new AbstractFileSystem object that will then create a new dfsClient which will create a new rpcProxy which will do a getVersion() - this is an extra round trip. Good news is that the connection at the lowest layer is cached. 

We can fix this by having each rpc call also piggy back the version number so that the version check is not needed when a proxy is created. This however will break wire compatibility.  

> Add a Cache for AbstractFileSystem in the new FileContext/AbstractFileSystem framework.
> ---------------------------------------------------------------------------------------
>
>                 Key: HADOOP-6356
>                 URL: https://issues.apache.org/jira/browse/HADOOP-6356
>             Project: Hadoop Common
>          Issue Type: Improvement
>          Components: fs
>    Affects Versions: 0.22.0
>            Reporter: Sanjay Radia
>            Assignee: Sanjay Radia
>             Fix For: 0.22.0
>
>
> The new filesystem framework, FileContext and AbstractFileSystem does not implement a cache for AbstractFileSystem.
> This Jira proposes to add a cache to the new framework just like with the old FileSystem.

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