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https://issues.apache.org/jira/browse/HADOOP-15999?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16793031#comment-16793031
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Steve Loughran commented on HADOOP-15999:
-----------------------------------------
really close to getting in, ran lots of tests, am happy. I tried adding a new
test but failed and gave up HADOOP-16193 is the outcome there.
One more change to request: skip going to s3 if the file checked is a
directory. Because if the dest is also a directory, there's no difference.
Pro: misses out the two failing HEAD calls and an expensive LIST whose output
is discarded
Con: doesn't catch up on the special failure case: someone has taken a
directory path /a/b/ and overwritten it with a file /a/b .
'
If we did want to worry about that, then rather than doing the whole
s3GetFileStatus call, we only need to execute a single getObjectMetadata for
the key "a/b" and, if something is actually there do an update.
That would still be hitting the store, but it'd only be doing 1/3 as many
requests
> S3Guard: Better support for out-of-band operations
> --------------------------------------------------
>
> Key: HADOOP-15999
> URL: https://issues.apache.org/jira/browse/HADOOP-15999
> Project: Hadoop Common
> Issue Type: Sub-task
> Components: fs/s3
> Affects Versions: 3.1.0
> Reporter: Sean Mackrory
> Assignee: Gabor Bota
> Priority: Major
> Attachments: HADOOP-15999-007.patch, HADOOP-15999.001.patch,
> HADOOP-15999.002.patch, HADOOP-15999.003.patch, HADOOP-15999.004.patch,
> HADOOP-15999.005.patch, HADOOP-15999.006.patch, HADOOP-15999.008.patch,
> HADOOP-15999.009.patch, out-of-band-operations.patch
>
>
> S3Guard was initially done on the premise that a new MetadataStore would be
> the source of truth, and that it wouldn't provide guarantees if updates were
> done without using S3Guard.
> I've been seeing increased demand for better support for scenarios where
> operations are done on the data that can't reasonably be done with S3Guard
> involved. For example:
> * A file is deleted using S3Guard, and replaced by some other tool. S3Guard
> can't tell the difference between the new file and delete / list
> inconsistency and continues to treat the file as deleted.
> * An S3Guard-ed file is overwritten by a longer file by some other tool. When
> reading the file, only the length of the original file is read.
> We could possibly have smarter behavior here by querying both S3 and the
> MetadataStore (even in cases where we may currently only query the
> MetadataStore in getFileStatus) and use whichever one has the higher modified
> time.
> This kills the performance boost we currently get in some workloads with the
> short-circuited getFileStatus, but we could keep it with authoritative mode
> which should give a larger performance boost. At least we'd get more
> correctness without authoritative mode and a clear declaration of when we can
> make the assumptions required to short-circuit the process. If we can't
> consider S3Guard the source of truth, we need to defer to S3 more.
> We'd need to be extra sure of any locality / time zone issues if we start
> relying on mod_time more directly, but currently we're tracking the
> modification time as returned by S3 anyway.
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