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From "Tsz Wo Nicholas Sze (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-7285) Erasure Coding Support inside HDFS
Date Tue, 27 Jan 2015 00:14:40 GMT

    [ https://issues.apache.org/jira/browse/HDFS-7285?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14292691#comment-14292691
] 

Tsz Wo Nicholas Sze commented on HDFS-7285:
-------------------------------------------

> ... but I guess we could also find many meanings for 'erasure' too. ...

Since it is under the io package, it is much harder to have an ambiguous meaning.  However,
io.ec still possibly means for error correcting (or arguably elliptic-curve cryptography).
 Won't you agree that a two-letter acronym "ec" is much more ambiguous than the word "erasure"?

> In this JIRA descrition for this effort, it's explicitly stated as below:

Yes, it is very clear in the current context since there is no other project for error correcting.
 But the package name sits there forever.  It we have a error correcting project later on,
it may become a problem.

> Our branch is also named HDFS-EC.

Branch is temporary and invisible to users, we may delete, rename and reuse branch name as
some point.  It is much harder to change package names since it is an incompatible change.

The "io.erasure" package name is just a minor suggestion.  I am fine if you insist using "io.ec"
although I think it may lead to some unnecessary confusion.

> 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: ECAnalyzer.py, ECParser.py, HDFSErasureCodingDesign-20141028.pdf,
HDFSErasureCodingDesign-20141217.pdf, fsimage-analysis-20150105.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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