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Date: Thu, 22 Apr 2010 14:29:52 -0400 (EDT)
From: "Kannan Muthukkaruppan (JIRA)" <jira@apache.org>
To: hbase-dev@hadoop.apache.org
Subject: [jira] Created: (HBASE-2478) Experiment with alternate settings for
 io.bytes.per.checksum for HFiles
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Experiment with alternate settings for io.bytes.per.checksum for HFiles
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                 Key: HBASE-2478
                 URL: https://issues.apache.org/jira/browse/HBASE-2478
             Project: Hadoop HBase
          Issue Type: Improvement
            Reporter: Kannan Muthukkaruppan


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).]


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