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From "Andre Araujo (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-12528) Short-circuit reads getting disabled frequently in certain scenarios
Date Thu, 21 Sep 2017 17:58:00 GMT
Andre Araujo created HDFS-12528:
-----------------------------------

             Summary: Short-circuit reads getting disabled frequently in certain scenarios
                 Key: HDFS-12528
                 URL: https://issues.apache.org/jira/browse/HDFS-12528
             Project: Hadoop HDFS
          Issue Type: Bug
          Components: hdfs-client, performance
    Affects Versions: 2.6.0
            Reporter: Andre Araujo


We have scenarios where data ingestion makes use of the -appendToFile operation to add new
data to existing HDFS files. In these situations, we're frequently running into the problem
described below.

We're using Impala to query the HDFS data with short-circuit reads (SCR) enabled. After each
file read, Impala "unbuffer"'s the HDFS file to reduce the memory footprint. In some cases,
though, Impala still keeps the HDFS file handle open for reuse.

The "unbuffer" call, however, causes the file's current block reader to be closed, which makes
the associated ShortCircuitReplica evictable from the ShortCircuitCache. When the cluster
is under load, this means that the ShortCircuitReplica can be purged off the cache pretty
fast, which closes the file descriptor to the underlying storage file.

That means that when Impala re-reads the file it has to re-open the storage files associated
with the ShortCircuitReplica's that were evicted from the cache. If there were no appends
to those blocks, the re-open will succeed without problems. If one block was appended since
the ShortCircuitReplica was created, the re-open will fail with the following error:

{code}
Meta file for BP-810388474-172.31.113.69-1499543341726:blk_1074012183_273087 not found
{code}

This error is handled as an "unknown response" by the BlockReaderFactory [1], which disables
short-circuit reads for 10 minutes [2] for the client.

These 10 minutes without SCR can have a big performance impact for the client operations.
In this particular case ("Meta file not found") it would suffice to return null without disabling
SCR. This particular block read would fall back to the normal, non-short-circuited, path and
other SCR requests would continue to work as expected.

It might also be interesting to be able to control how long SCR is disabled for in the "unknown
response" case. 10 minutes seems a bit to long and not being able to change that is a problem.

[1] https://github.com/apache/hadoop/blob/f67237cbe7bc48a1b9088e990800b37529f1db2a/hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/client/impl/BlockReaderFactory.java#L646

[2] https://github.com/apache/hadoop/blob/f67237cbe7bc48a1b9088e990800b37529f1db2a/hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/shortcircuit/DomainSocketFactory.java#L97




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