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https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12482494
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Raghu Angadi commented on HADOOP-1134:
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Regd Datanode upgrade : Online or offline?
Offline : When datanodes restart with the new version, they essentially go
offline for couple of hours (100+ GB) come up with a new and shiny blocks.
Pros:
1) Simpler code. upgrade code does not need to be maintained in
future versions. Users with with very old dfs could upgrade iteratively.
2) No change in block filenames or other maintenance is required.
Cons:
1) Cluster would in accessible for couple of hours which implies
lot of one time work especially admins.
2) Upgrade to future versions from current versions requires
multiple upgrades.
Online: Datanode would immediately start serving existing data after restart.
It will upgrade the blocks in background.
Pros:
1) no extra downtime.
Cons:
1) Increases the code and this exra code would be spread around the
datanode code.
2) The code to handle mixed database may not be removed anytime in
near future.
My (selfish) preference is to upgrade offline :). It does not seem unreasonable
before 1.0 release.
> 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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