[jira] [Commented] (HDFS-7068) Support multiple block placement policies

["Zhe Zhang (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HDFS-7068?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14596822#comment-14596822 ] 

Zhe Zhang commented on HDFS-7068:
---------------------------------

[~walter.k.su] Since we are preparing to merge the HDFS-7285 branch to trunk, we should probably revisit this JIRA. I suggest we split the HDFS-8186 patch and separate the multi-policy part out for this JIRA ({{BlockPlacementPolicies}} etc.). That part needs to be reviewed against trunk anyway as part of the merge. And logically it is orthogonal to EC logic. Separating it out will reduce the consolidated EC patch and make merge-review easier.

> Support multiple block placement policies
> -----------------------------------------
>
>                 Key: HDFS-7068
>                 URL: https://issues.apache.org/jira/browse/HDFS-7068
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: namenode
>    Affects Versions: 2.5.1
>            Reporter: Zesheng Wu
>            Assignee: Walter Su
>
> According to the code, the current implement of HDFS only supports one specific type of block placement policy, which is BlockPlacementPolicyDefault by default.
> The default policy is enough for most of the circumstances, but under some special circumstances, it works not so well.
> For example, on a shared cluster, we want to erasure encode all the files under some specified directories. So the files under these directories need to use a new placement policy.
> But at the same time, other files still use the default placement policy. Here we need to support multiple placement policies for the HDFS.
> One plain thought is that, the default placement policy is still configured as the default. On the other hand, HDFS can let user specify customized placement policy through the extended attributes(xattr). When the HDFS choose the replica targets, it firstly check the customized placement policy, if not specified, it fallbacks to the default one. 
> Any thoughts?



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