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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1134) Block level CRCs in HDFS
Date Fri, 01 Jun 2007 00:02:16 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12500532
] 

Raghu Angadi commented on HADOOP-1134:
--------------------------------------


bq. The total size of the packet in bytes. Having this up front might make it easier to, e.g.,
write an NIO-based datanode that uses async io. Ideally we could re-write datanode to be async
without modifying the on-the-wire protocol.

I am still not clear which length is missing. Length of common header is a constant. Both
OP_READ_BLOCK and OP_WRITE_BLOCK include lengths. In the case of WRITE, html doc is out of
date. I will update.

bq. I don't see the use case for transmitting the start and length with each checksum, rather
it seems like it only makes sense once per request, no? So why not factor it to the OP-level?

E.g. OP_READ_BLOCK:
Right now start_offset is required for the first 'DATA_CHUNK' and length is required for last
two DATA_CHUNKS (at least one data chunk for sure) to indicate end of the stream (for what
ever reason). Using 'Vint' will bring the byte over head to 5-6. So start_offset can be removed
from DATA_CHUNK. I would prefer to keep the length so that loops that read and write to these
streams could be a little simpler.





> 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
>            Assignee: Raghu Angadi
>         Attachments: bc-no-upgrade-05302007.patch, DfsBlockCrcDesign-05305007.htm
>
>
> 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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