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https://issues.apache.org/jira/browse/HDFS-12528?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16340558#comment-16340558
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Xiao Chen commented on HDFS-12528:
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Thanks for looking at this [~GeLiXin] and offering to work on it.
I only did #1 because that's easiest and the most compatible way. Similar to #2
being subjective, #3 also varies based on the exception. It may turn out in the
future that some other exceptions should not disable SCR too, which will
require more change. But if you want to give #3 a try, please feel free to, I
can help with reviews then. If that takes time we can also separate out a new
jira.
[~jzhuge] and [~cheersyang], any thoughts from you?
> Short-circuit reads unnecessarily disabled for a long time
> ----------------------------------------------------------
>
> 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
> Assignee: Xiao Chen
> Priority: Major
> Attachments: HDFS-12528.000.patch, HDFS-12528.01.patch
>
>
> 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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