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From "Jason Lowe (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-8182) Implement topology-aware CDN-style caching
Date Wed, 22 Apr 2015 21:48:00 GMT

    [ https://issues.apache.org/jira/browse/HDFS-8182?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14507982#comment-14507982
] 

Jason Lowe commented on HDFS-8182:
----------------------------------

Quota seems like it could be problematic for some of the use-cases involved in the distributed
cache.  Not all files being downloaded by a user belong to that user.  Public localized resources
are a good example of this.  How does quota enter the picture in the case where the user who
owns the file is not the user requesting the file?  For example, are other users accessing
my public files going to be able to cause my quota usage to increase by this feature?

> Implement topology-aware CDN-style caching
> ------------------------------------------
>
>                 Key: HDFS-8182
>                 URL: https://issues.apache.org/jira/browse/HDFS-8182
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: hdfs-client, namenode
>    Affects Versions: 2.6.0
>            Reporter: Gera Shegalov
>
> To scale reads of hot blocks in large clusters, it would be beneficial if we could read
a block across the ToR switches only once. Example scenarios are localization of binaries,
MR distributed cache files for map-side joins and similar. There are multiple layers where
this could be implemented (YARN service or individual apps such as MR) but I believe it is
best done in HDFS or even common FileSystem to support as many use cases as possible. 
> The life cycle could look like this e.g. for the YARN localization scenario:
> 1. inputStream = fs.open(path, ..., CACHE_IN_RACK)
> 2. instead of reading from a remote DN directly, NN tells the client to read via the
local DN1 and the DN1 creates a replica of each block.
> When the next localizer on DN2 in the same rack starts it will learn from NN about the
replica in DN1 and the client will read from DN1 using the conventional path.
> When the application ends the AM or NM's can instruct the NN in a fadvise DONTNEED style,
it can start telling DN's to discard extraneous replica.



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