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https://issues.apache.org/jira/browse/HBASE-27896?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Bryan Beaudreault resolved HBASE-27896.
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Fix Version/s: 2.6.0
3.0.0-beta-1
Release Note: PREAD reads will no longer do HDFS readahead by default.
This should save substantial disk and network IO for random read workloads, but
one can re-enable it if desired by setting "hbase.store.reader.no-readahead" to
false.
Assignee: Bryan Beaudreault
Resolution: Fixed
Pushed to branch-2, branch-3, and master. Thanks everyone for chiming in and
[~reidchan] for the review.
See [https://lists.apache.org/thread/pokw1bwtr26hdbmlmx4tx1g1fczqrtxt] for the
discussion thread.
> Disable hdfs readahead for pread reads
> --------------------------------------
>
> Key: HBASE-27896
> URL: https://issues.apache.org/jira/browse/HBASE-27896
> Project: HBase
> Issue Type: Improvement
> Reporter: Bryan Beaudreault
> Assignee: Bryan Beaudreault
> Priority: Major
> Fix For: 2.6.0, 3.0.0-beta-1
>
>
> In https://issues.apache.org/jira/browse/HBASE-17914, a flag was introduced
> {{{}hbase.store.reader.no-readahead{}}}. The default is false, so readahead
> is enabled. This flag is used for creating the default store reader (i.e. the
> one used by PREAD reads). Stream readers don't use this flag, instead they
> always pass -1.
> When that flag is true, we pass a readahead value of 0 to
> FSDataInputStream.setReadahead. When the flag is false, we pass -1 which
> triggers hdfs default behavior. The default behavior is to use a readahead of
> 4MB.
> It seems to me that we don't want readahead for PREAD reads, and especially
> not such a large readahead. Our default block size is 64kb, which is much
> smaller than that. A PREAD read is not likely to do sequential IO, so not
> likely to utilize the cached readahead buffer.
> I set no-readahead to true in a few of our clusters and in each case saw a
> massive reduction in disk IO and thus increase in throughput. I load tested
> this in a test cluster which does fully random reads of ~300 byte rows on a
> dataset which is 20x larger than memory. The load test was able to achieve
> nearly double the throughput.
> As a follow-on, we might consider tuning the readahead for STREAM reads. 4mb
> seems way too big for many common workloads.
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