hadoop-common-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1134) Block level CRCs in HDFS
Date Thu, 31 May 2007 20:23:15 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12500476
] 

Doug Cutting commented on HADOOP-1134:
--------------------------------------

Re the patch: As I've mentioned before, I'd prefer it if the CRC code could be shared with
CheckSumFileSystem.  In particular, it seems to me that FSInputChecker and FSOutputSummer
could be extended to support pluggable sources and sinks for checksums, respectively, and
DFSDataInputStream and DFSDataOutputStream could use these.  Advantages of this are: (a) single
implementation of checksum logic to debug and maintain; (b) keeps checksumming as close to
possible to data generation and use.  This patch computes checksums after data has been buffered,
and validates them before it is buffered.   We sometimes use large buffers and would like
to guard against in-memory errors.  The current checksum code catches a lot of such errors.
 So we should compute checksums after minimal buffering (just bytesPerChecksum, ideally) and
validate them at the last possible moment (e.g., through the use of a small final buffer with
a larger buffer behind it).  I do not think this will significantly affect performance, and
data integrity is a high priority.

> 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.
>  

-- 
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.


Mime
View raw message