[
https://issues.apache.org/jira/browse/HBASE-2478?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12885972#action_12885972
]
Todd Lipcon commented on HBASE-2478:
------------------------------------
Would be interesting to run these comparisons with the new PureJavaCrc32
integrated into the Hadoop build. It changes the performance profile of
checksumming pretty significantly, and since it's in 0.21 (and a trivial patch
to put in 0.20) we might as well test with it.
(The JIRAs are HADOOP-6148, HDFS-496, HADOOP-6166)
> Experiment with alternate settings for io.bytes.per.checksum for HFiles
> -----------------------------------------------------------------------
>
> Key: HBASE-2478
> URL: https://issues.apache.org/jira/browse/HBASE-2478
> Project: HBase
> Issue Type: Improvement
> Reporter: Kannan Muthukkaruppan
> Assignee: Hairong Kuang
>
> HDFS keeps a separate "checksum" file for every block. By default,
> io.bytes.per.checksum is set at 512, and the checksums are 4 bytes... i.e.
> for every 512 bytes of data in the block we maintain a 4 byte checksum. For
> 4TB of data, for instance, that's about 31GB of checksum data.
> A read that needs to read a small section (such as a 64k HFile block) from a
> HDFS block, especially on a cold access, is likely to end up doing two random
> disk reads--- one from the data file for the block and one from the checksum
> file.
> A though was that instead of keeping a checksum for every 512 bytes, given
> that HBase will interact with HDFS on reads at the granularity of HBase block
> size (typically 64k, but smaller if compressed), should we consider keeping
> checksums at a coarser granularity (e.g, for every 8k bytes) for HFiles? The
> advantage
> with this would be that the checksum files would be much smaller (in
> proportion to the data) and the hot working set for "checksum data" should
> fit better in the OS buffer cache (thus eliminating a good majority of the
> disk seeks for checksum data).
> The intent of the JIRA is to experiment with different settings for
> "io.bytes.per.checksum" for HFiles.
> Note: For the previous example, of 4TB of data, with an io.bytes.per.checksum
> setting of 8k, the size of the checksum data would drop to about 2Gig.
> Making the io.bytes.per.checksum too big might reduce the effectiveness of
> the checksum. So that needs to be taken into account as well in terms of
> determining a good value.
> [For HLogs files, on the other hand, I suspect we would want to leave the
> checksum at finer granularity because my understanding is that if we are
> doing lots of small writes/syncs (as we do to HLogs), finer grained checksums
> are better (because the code currently doesn't do a rolling checksum, and
> needs to rewind to the nearest checksum block boundary and recomputed the
> checksum on every edit).]
--
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.
