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https://issues.apache.org/jira/browse/HDFS-2699?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13172059#comment-13172059
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M. C. Srivas commented on HDFS-2699:
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@Todd: no one is arguing that putting the CRC inline is not beneficial wrt seek
time. Recalculating CRC over with a 4K block is substantially slower than with
a 512-byte block (256 bytes vs 2K on the average is a 10x factor). Imagine
appending continuously to the HBase WAL with the 128-byte records that you
mentioned in another thread ... the CPU burn will be much worse with 4K CRC
blocks.
Secondly, the disk manufacturers guarantee only a 512-byte atomicity on disk.
Linux doing a 4K block write guarantees almost nothing wrt atomicity of that 4K
write to disk. On a crash, unless you are running some sort of RAID or
data-journal, there is a likelihood of the 4K block that's in-flight getting
corrupted.
> Store data and checksums together in block file
> -----------------------------------------------
>
> Key: HDFS-2699
> URL: https://issues.apache.org/jira/browse/HDFS-2699
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: dhruba borthakur
> Assignee: dhruba borthakur
>
> The current implementation of HDFS stores the data in one block file and the
> metadata(checksum) in another block file. This means that every read from
> HDFS actually consumes two disk iops, one to the datafile and one to the
> checksum file. This is a major problem for scaling HBase, because HBase is
> usually bottlenecked on the number of random disk iops that the
> storage-hardware offers.
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