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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1134) Block level CRCs in HDFS
Date Mon, 09 Jul 2007 21:15:04 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12511258

Doug Cutting commented on HADOOP-1134:

> Even if I use FSOutputSummer(), I still need this since DFSOutputStream need to override
write() any way (in each write, it needs decide if we need to close the burrent block and
open a new one).

I don't follow.  If block sizes are always a multiple of bytesPerChecksum, as we agreed, then
a chunk should never cross a block boundary, right?

Also, using a one-byte-buffer may still impact performance, since it must call multiple methods
per call to write(int).  It can make a significant performance difference if write(int) does
much more than check a buffer boundary and buffer[count++] = byte.

> 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: BlockLevelCrc-07032007.patch, BlockLevelCrc-07052007.patch, BlockLevelCrc-07062007.patch,
BlockLevelCrc-07062007.patch, BlockLevelCrc-07062007.patch, DfsBlockCrcDesign-05305007.htm,
> 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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