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From "Konstantin Shvachko (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1134) Block level CRCs in HDFS
Date Thu, 29 Mar 2007 03:12:26 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12485051
] 

Konstantin Shvachko commented on HADOOP-1134:
---------------------------------------------

Sorry, joining the discussion on such a late stage. There's been a lot of progress here. Got
some fresh thoughts :-)

Currently the name-node does not know anything about .crc files. IMO we should keep it that
way or at least minimize the impact.
That said, I'd not support the idea of implementing the getChecksumAuthority() method on the
name-node, since if we do
it the name-node will act as a client to itself and its data-nodes.
I would propose to implement a separate crcConverter - a client, which acts as an fsck client
as we used to have in the past.
We can make it MR-distributed if we want to accelerate things.
The crcConverter
- reads a set of files and checks each block determining which crc block is valid, basically
implementing
Sameer's getChecksumAuthority() but on the client.
- Then asks each data-node to takeChecksumAuthority() over the block by either copying its
crc from another node or generating it locally.
- And then removes the crc file, which will also lead to removal of old crc blocks.

In case of failure the crcConverter will restart as of nothing has been done before.
For those files that have already been converted the converter will not find its crc files
and will ask the data-nodes to
takeChecksumAuthority() over the corresponding blocks by generating them locally. Data-nodes
will see that corresponding
crc files have already been generated and will do nothing.

With respect to the name-node, during crc conversion it should be started with -upgrade option,
and the converter should wait
until everything is upgraded, replicated and stabilized on the cluster. Then it can enter
the manual safe mode and do the conversion.
I am sure there is a lot of details missing in my proposal, but it seems simpler to me because
it requires less changes on the name- and data-nodes.

> 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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