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From "Yi Liu (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (HDFS-7285) Erasure Coding Support inside HDFS
Date Sun, 01 Feb 2015 07:39:35 GMT

    [ https://issues.apache.org/jira/browse/HDFS-7285?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14300105#comment-14300105
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Yi Liu edited comment on HDFS-7285 at 2/1/15 7:38 AM:
------------------------------------------------------

{quote}
Good catch. I need to update that design doc to match the 64KB default stripe cell size. BTW
1MB is the I/O buffer size (see parameters C and B on page 8 of the master design doc 
{quote}
I think we need to  allow dynamic stripe cell size depends on the file size. If we only use
small fixed value, for example 64KB as the stripe cell size, then for large file, we need
much more ec block groups to store the entire file than the number of blocks we need using
replication way, even as implemented in HDFS-7339, we only store the first ec block of the
ec block groups in NN, but the NN memory consumption is a big issue since there are too many
ec block groups. 


was (Author: hitliuyi):
{quote}
Good catch. I need to update that design doc to match the 64KB default stripe cell size. BTW
1MB is the I/O buffer size (see parameters C and B on page 8 of the master design doc 
{quote}
I think we need to  allow dynamic stripe cell size depends on the file size. If we only use
small fixed value, for example 64KB as the stripe cell size, then for large file, we need
much more ec block groups to store the entire file than using replication way, even as implemented
in HDFS-7339, we only store the first ec block of the ec block groups in NN, but the NN memory
consumption is a big issue since there are too many ec block groups. 

> 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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