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From "Yongjun Zhang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-7285) Erasure Coding Support inside HDFS
Date Fri, 31 Oct 2014 20:53:36 GMT

    [ https://issues.apache.org/jira/browse/HDFS-7285?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14192471#comment-14192471
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Yongjun Zhang commented on HDFS-7285:
-------------------------------------

Hi Zhe,

Thanks for organizing the meeting, and thanks all the folks for attending, it's a good one!

One thing I forgot to mention, we need to add a section in the design spec about the impact
to distcp. There are multiple things to be addressed (I do not mean to make the list complete):
* copy between two clusters, one with EC support and one without, 
* should we restore data before copying or just copying the coded blocks.  The latter may
not be feasible since the coded data may contain other files that are not to be copied; Even
if the coded blocks belongs to the file to be copied, copying it to a target directory not
tagged as EC  means some handling there. And copying to a cluster not supporting EC need special
care too. 
* distcp has switch to preserve block size, if this switch is not specified, a different block
size may be used at target, we need special handling here.
...

Thanks.


> Erasure Coding Support inside HDFS
> ----------------------------------
>
>                 Key: HDFS-7285
>                 URL: https://issues.apache.org/jira/browse/HDFS-7285
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>            Reporter: Weihua Jiang
>            Assignee: Zhe Zhang
>         Attachments: HDFSErasureCodingDesign-20141028.pdf
>
>
> Erasure Coding (EC) can greatly reduce the storage overhead without sacrifice of data
reliability, comparing to the existing HDFS 3-replica approach. For example, if we use a 10+4
Reed Solomon coding, we can allow loss of 4 blocks, with storage overhead only being 40%.
This makes EC a quite attractive alternative for big data storage, particularly for cold data.

> Facebook had a related open source project called HDFS-RAID. It used to be one of the
contribute packages in HDFS but had been removed since Hadoop 2.0 for maintain reason. The
drawbacks are: 1) it is on top of HDFS and depends on MapReduce to do encoding and decoding
tasks; 2) it can only be used for cold files that are intended not to be appended anymore;
3) the pure Java EC coding implementation is extremely slow in practical use. Due to these,
it might not be a good idea to just bring HDFS-RAID back.
> We (Intel and Cloudera) are working on a design to build EC into HDFS that gets rid of
any external dependencies, makes it self-contained and independently maintained. This design
lays the EC feature on the storage type support and considers compatible with existing HDFS
features like caching, snapshot, encryption, high availability and etc. This design will also
support different EC coding schemes, implementations and policies for different deployment
scenarios. By utilizing advanced libraries (e.g. Intel ISA-L library), an implementation can
greatly improve the performance of EC encoding/decoding and makes the EC solution even more
attractive. We will post the design document soon. 



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