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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-503) Implement erasure coding as a layer on HDFS
Date Thu, 10 Sep 2009 00:53:57 GMT

    [ https://issues.apache.org/jira/browse/HDFS-503?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12753349#action_12753349
] 

Raghu Angadi commented on HDFS-503:
-----------------------------------

This seems pretty useful.  since this is done outside HDFS, it is simpler for users to start
experimenting.

Say a file has 5 blocks with replication of 3 : total 15 blocks
With this tool, replication could be reduced to 2, with one block for parity : total 10 +
2 blocks
This is a savings of 20% space. Is this math correct?

Detecting when to  'unRaid' : 
  * The patch does this using a wrapper filesystem over HDFS.
      ** This requires file to be read by the client. 
      ** More often than not, HDFS knows about irrecoverable blocks much before a client 
reads.
      ** this only semi-transparent to the users since they have to use the new filesystem.

  * Another completely transparent alternative could be to make 'RaidNode' ping NameNode for
missing blocks.
      ** NameNode already knows about blocks that don't have any known good replica. And fetching
that list is cheap.
      ** RaidNode could check if the corrupt/missing block belongs to any of its files. 
      ** Rest of RaidNode pretty much remains the same as this patch.

> Implement erasure coding as a layer on HDFS
> -------------------------------------------
>
>                 Key: HDFS-503
>                 URL: https://issues.apache.org/jira/browse/HDFS-503
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>            Reporter: dhruba borthakur
>            Assignee: dhruba borthakur
>         Attachments: raid1.txt
>
>
> The goal of this JIRA is to discuss how the cost of raw storage for a HDFS file system
can be reduced. Keeping three copies of the same data is very costly, especially when the
size of storage is huge. One idea is to reduce the replication factor and do erasure coding
of a set of blocks so that the over probability of failure of a block remains the same as
before.
> Many forms of error-correcting codes are available, see http://en.wikipedia.org/wiki/Erasure_code.
Also, recent research from CMU has described DiskReduce https://opencirrus.org/system/files/Gibson-OpenCirrus-June9-09.ppt.
> My opinion is to discuss implementation strategies that are not part of base HDFS, but
is a layer on top of HDFS.

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