[
https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12485321
]
Sameer Paranjpye commented on HADOOP-1134:
------------------------------------------
As Konstantin suggests, using a client program to perform validation is also
reasonable. It has the advantage of keeping upgrade code in HDFS very simple
and decoupling the Namenode and Datanode upgrades.
Datanodes would perform local upgrades during which they'd re-generate
checksums for all their blocks and put them in side checksum files. Once this
is done, we could lauch a Map/Reduce job that reads data files and validates
them against the existing .crc files, ensuring that it reads all blocks. If it
discovers corruption, it reports the corrupt blocks to the Namenode, which can
then proceed to invalidate them and replicate the correct instances. For every
file that is successfully validated the client would delete the .crc file from
the namespace. Dealing with missing replicas is a bit tricky with this
approach, the other downside it has is that it is potentially much slower since
validating with Map/Reduce could cause a lot of data transfer over the network.
> 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.
>
--
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.
