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From "Sameer Paranjpye (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1134) Block level CRCs in HDFS
Date Wed, 28 Mar 2007 18:34:25 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12484957

Sameer Paranjpye commented on HADOOP-1134:

This is complicated. The general design direction appears to be the right one, but I think
some key details need to be spelled  out.

We say we're going to use the default of 512 bytes/chksum. What do we do for HDFS installations
that use a different value in configuration? What do we do for installations that have used
more than one value of bytes/chksum when generating data?

One option is to simply use the existing checksum data, as is, with the understanding that
we could end up with different values of bytes/chksum across HDFS installations and across
different files in the same installation. The alternative would be to re-generate checksum
data on the Datanodes with 512 bytes/chksum and validate the new checksums against the existing

Do we keep the io.bytes.per.checksum configuration variable or do we kill it? 

Do we simply copy existing checksum data or do we re-generate it?

I don't think simply copying checksum data is enough since the checksums can themselves be
corrupt. We need some level of validation. We can compare copies of the checksum data against
each other, if we find a majority of copies that match then we treat those as authoritative.
But what happens when we don't find a majority? Or we can re-generate checksum data on the
Datanode and validate it against the existing data.

How does a Datanode discover authoritative sources of checksum data for it's blocks?

This is presumably done with a call to the Namenode that given a block id responds with the
name of a checksum file. The Datanode then reads the header, determines the offset and length
where the specified blocks checksums lie, then reads the checksum data and validates it. This
works while the upgrade is in progress but perhaps it can be extended to deal with Datanodes
that join the system after the upgrade is complete. If a Datanode joins after a complete upgrade
and crc file deletion, the Namenode could redirect it to other Datanodes that have copies
of it's blocks, the new Datanode can then pull block level CRC files from it's peers, validate
it's data and  perform an upgrade even though the .crc files are gone.


> 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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