Virajith Jalaparti commented on HDFS-9806:

Hi [~mackrorysd], If this feature is included but not enabled, the only change 
in the memory consumed is that of allocating one new object (for a 
{{ProvidedStorageMap}}). So, you wouldn't see any change in the memory consumed 
by the Namenode.

As far as scalability when this feature is enabled is concerned, it should 
scale (in terms of blocks and files) like the Namenode without this feature. To 
figure out the state that needs to be maintained by the Namenode for files in 
external storage, you can think about them exactly like files that are local to 

> Allow HDFS block replicas to be provided by an external storage system
> ----------------------------------------------------------------------
>                 Key: HDFS-9806
>                 URL: https://issues.apache.org/jira/browse/HDFS-9806
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>            Reporter: Chris Douglas
>         Attachments: HDFS-9806-design.001.pdf, HDFS-9806-design.002.pdf
> In addition to heterogeneous media, many applications work with heterogeneous 
> storage systems. The guarantees and semantics provided by these systems are 
> often similar, but not identical to those of 
> [HDFS|https://hadoop.apache.org/docs/current/hadoop-project-dist/hadoop-common/filesystem/index.html].
>  Any client accessing multiple storage systems is responsible for reasoning 
> about each system independently, and must propagate/and renew credentials for 
> each store.
> Remote stores could be mounted under HDFS. Block locations could be mapped to 
> immutable file regions, opaque IDs, or other tokens that represent a 
> consistent view of the data. While correctness for arbitrary operations 
> requires careful coordination between stores, in practice we can provide 
> workable semantics with weaker guarantees.

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