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https://issues.apache.org/jira/browse/HDFS-12528?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16301870#comment-16301870
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Xiao Chen commented on HDFS-12528:
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Looked at this again with the helpful unit test from John.
IMO we can do one or some of the following:
# Make the {{expireAfterWrite}} of {{DomainSocketFactory$pathMap}} configurable.
This is lowest risk and gives clients the freedom. {{getPathInfo}} will return
{{VALID}} state if the entry is not found from {{pathMap}}, so setting it to 0
basically never disables the domain socket.
# Add a time based counter on {{DomainSocketFactory}} and only disable after a
configurable number of errors seen within an interval (say, 10 errors in 10
mins?) for a given path.
This may be more smarter and wouldn't need the basic users to change the config.
Trade off is if a problematic block is read very frequently we'll still have
this same issue.
# Add a field about exception type to {{BlockOpResponseProto}} (or something
morphs). Anecdotally the reports here are all FNFE. We can update
BlockReaderFactory to ignore unknown errors that are FNFE, but still disable
other unknown errors.
This will change more stuff, and trivially change the behavior. Still
compatible though.
In any case, upon the null {{BlockReader}} from SCR, the read will fall back to
regular RPC.
I'm proposing we do #1 and #2. Thoughts?
> 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
> 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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