[
https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12484249
]
Raghu Angadi commented on HADOOP-1134:
--------------------------------------
> I thought we'd keep this at 512B until we'd benchmarked things?
64KB is only for new blocks. We will keep the 512 byte checksums during the
upgrade.
> Aren't we going to simply copy the existing checksums, rather than
> re-generate new checksums or verify against existing? That'd be a lot faster.
You are right. I missed that. will correct it. We will just read the existing
CRCs.
>> DN verifies the checksum each time it reads the block data
> I thought that only the client would validate, reporting failures to the DN.
> Why validate twice, since we have to validate in the client anyway?
I am not sure of this. I personally prefer datanode verification (and general
ownership of CRCs). I don't think it adds noticeable latency.. I think CPU
overhead would be reasonable.
> 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.
