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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1134) Block level CRCs in HDFS
Date Mon, 02 Apr 2007 23:08:32 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12486164

Raghu Angadi commented on HADOOP-1134:

Sameer talked to me about the following approach for upgrade. The following is based on the
discussion :

1) Namenode consideres upgrade complete only when 100% of blocks have at least one replica
upgraded (as described below). We will still  have a manual override call to mark upgrade
complete (i.e. finalizeUpgrade() call that was mentioned earlier).

2) During upgrade, datanodes fetch all the replicas of CRC data (C1, C2, C3.. ) for a given
block B. If the CRC replicas match, it stores generates new CRC file with the data. There
is no verification with the block data in this case. If not all CRCs match but majority of
them match, then majority CRC  will be used. If all are different then, we just pick one (with
least block id?).

3) If there is no existing CRC data available for some reason, we generate from existing block
data. A warning will be printed.

4) If a datanode comes into cluster with old data directories and notices that namenode has
completed upgrade, it upgrades each of the blocks like this:
    4.a) It gets the datanode that has an 'upgraded replica' for a block and contacts that
datanode for the CRC data.
    4.b) If can not find an upgraded replica, it generates local CRC.. Or should it delete
    4.c) It waits for some time (half an hour) before deciding if an upgraded replica is missing.

> 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
>         Assigned To: Raghu Angadi
> 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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