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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1134) Block level CRCs in HDFS
Date Thu, 21 Jun 2007 22:12:26 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12507049
] 

Raghu Angadi commented on HADOOP-1134:
--------------------------------------

Brief out line of when an upgrade is considered complete and how a manual override works :

  * Upgrade process starts when normal safemode conditions are met.
  * Any datanode that is already registered or heartbeats after the upgrade starts will be
asked to perform datanode upgrade.
  * Without manual override, Namenode waits for *all* the nodes that were known to complete
their upgrade.
  * Every few minutes it prints a brief message in namenode log about the current status.

  * The brief report lists the datanode that have not finished if there are only a handful
left (may be <= 10?).
  * If some datanodes go down after the upgrade starts, automatic upgrade might not finish
at all

Manual override:

   * When an admin notices an upgrade that seems to be stuck, a 'detailed report' can be requests.
   * A detailed report iterates through all the blocks and checks how many blocks belong to
following categories :
   *# Atleast minReplicas placed on upgraded datanodes.
   *#  All the replicas placed on upgraded nodes.
   *# None of the replicas placed on upgraded nodes.
   * Based on  above data, an admin can decide either manually stop the upgrade (if there
are no blocks with zero upgraded replicas) or alternately bring up a dead datanode.
   * Detailed report and manual override can be done through new dfsadmin commands that will
 be added.
 


> Block level CRCs in HDFS
> ------------------------
>
>                 Key: HADOOP-1134
>                 URL: https://issues.apache.org/jira/browse/HADOOP-1134
>             Project: Hadoop
>          Issue Type: New Feature
>          Components: dfs
>            Reporter: Raghu Angadi
>            Assignee: Raghu Angadi
>         Attachments: bc-no-upgrade-05302007.patch, DfsBlockCrcDesign-05305007.htm, readBuffer.java,
readBuffer.java
>
>
> Currently CRCs are handled at FileSystem level and are transparent to core HDFS. See
recent improvement HADOOP-928 ( that can add checksums to a given filesystem ) regd more about
it. Though this served us well there a few disadvantages :
> 1) This doubles namespace in HDFS ( or other filesystem implementations ). In many cases,
it nearly doubles the number of blocks. Taking namenode out of CRCs would nearly double namespace
performance both in terms of CPU and memory.
> 2) Since CRCs are transparent to HDFS, it can not actively detect corrupted blocks. With
block level CRCs, Datanode can periodically verify the checksums and report corruptions to
namnode such that name replicas can be created.
> We propose to have CRCs maintained for all HDFS data in much the same way as in GFS.
I will update the jira with detailed requirements and design. This will include same guarantees
provided by current implementation and will include a upgrade of current data.
>  

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