Steve Loughran created HADOOP-17597:
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Summary: Add option to downgrade S3A rejection of Syncable to
warning + iostatistics
Key: HADOOP-17597
URL: https://issues.apache.org/jira/browse/HADOOP-17597
Project: Hadoop Common
Issue Type: Bug
Affects Versions: 3.3.1
Reporter: Steve Loughran
Assignee: Steve Loughran
The Hadoop Filesystem Syncable API is intended to meet the requirements laid
out in [StoneBraker81] _Operating System Support for Database Management_
bq. The service required from an OS buffer manager is a selectedforce out
which would push the intentions list and the commit flag to disk in the proper
order. Such a service is not present in any buffer manager known to us.
It's an expensive operation -so expensive that {{Syncable.hsync()}} isn't even
called on {{DFSOutputStream.close()}}. I
Even though S3A does not manifest any data until close() is called,
applications coming from HDFS may call Syncable methods and expect to them to
persist data with the durability guarantees offered by HDFS.
Since the output stream hardening of HADOOP-13327, S3A throws
UnsupportedOperationException to indicate that the synchronization semantics of
Syncable absolutely cannot be met.
As a result, applications which have been calling the Syncable APIs are finding
the call failing. In the absence of exception handling to recognise that the
durability semantics are being met, they fail.
If the user and the application actually expects data to be persisted, this is
the correct behaviour. The data cannot be persisted this way.
If, however, they were calling this on HDFS more as a {{flush()}} than the full
and expensive DBMS-class persistence call, then this failure is unwelcome. The
applications really needs to catch the UnsupportedOperationException raised by
S3A _or any other FS strictly reporting failures_, report the problem and
perform some other means of safe data storage
Even better, they can use hasPathCapability on the FS or hasCapability() on the
stream to probe before even opening a file or trying to sync it. the
hasCapability() on a stream was actually implemented in Hadooop-2.x precisely
to allow applications to identify when a stream could not meet the guarantees
(e.g some of the encrypted streams, file:// before HADOOP-13...)
Until they can correct their code, I propose adding the option for s3a to
downgrade
fs.s3a.downgrade.syncable.exceptions
This will
* Log once per process at WARN
* downgrade the calls to noop()
* increment counters in S3A stats and IO stats of invocations of the Syncable
methods. This will allow for stats gathering to let us identify which
applications need fixing in cloud deployments
Testing: copy the hsync tests but expect exceptions to be swallowed and stats
to be collected
Also: UnsupportedException text will link to this JIRA
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