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https://issues.apache.org/jira/browse/HDFS-12528?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16222326#comment-16222326
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Weiwei Yang commented on HDFS-12528:
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I believe we met the same issue, but triggered by a different cause. Our case a
hbase client is trying to SCR a block that has been moved away by balancer, it
complains that the block replica not valid, but this is an unknown error
{noformat}
impl.BlockReaderFactory: BlockReaderFactory(fileName=xxx,
block=BP-547663139-11.139.225.193-1497349178310:blk_2222386301_1148917283):
unknown response code ERROR while attempting to set up short-circuit access.
Block BP-xxx:blk_2222386301_1148917283 is not valid
{noformat}
this causes the SCR been disabled for 10 minutes and impacted hbase performance
a lot. [~jzhuge], how do you plan to fix this? I guess this long pause was in
case something really bad happens. But in real use cases, we might keep seeing
new exceptions (unknown) but don't prefer to disable SCR for so long (like
here). Can we have this to be configurable length of time? And if we set it to
0, that gives us a way to NOT disable SCR at all.
Thanks
> 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
> Assignee: John Zhuge
> Attachments: HDFS-12528.000.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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