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From Andrew Wang <andrew.w...@cloudera.com>
Subject Re: Feature request to provide DFSInputStream subclassing mechanism
Date Thu, 08 Aug 2013 02:47:08 GMT
Blocks are supposed to be an internal abstraction within HDFS, and aren't
an inherent part of FileSystem (the user-visible class used to access all
Hadoop filesystems).

Is it possible to instead deal with files and offsets? On a read failure,
you could open a stream to the same file on the backup filesystem, seek to
the old file position, and retry the read. This feels like it's possible
via wrapping.

On Wed, Aug 7, 2013 at 3:29 PM, Jeff Dost <jdost@ucsd.edu> wrote:

> Thank you for the suggestion, but we don't see how simply wrapping a
> FileSystem object would be sufficient in our use case.  The reason why is
> we need to catch and handle read exceptions at the block level.  There
> aren't any public methods available in the high level FileSystem
> abstraction layer that would give us the fine grained control we need at
> block level read failures.
> Perhaps if I outline the steps more clearly it will help explain what we
> are trying to do.  Without our enhancements, suppose a user opens a file
> stream and starts reading the file from Hadoop. After some time, at some
> position into the file, if there happen to be no replicas available for a
> particular block for whatever reason, datanodes have gone down due to disk
> issues, etc. the stream will throw an IOException (BlockMissingException or
> similar) and the read will fail.
> What we are doing is rather than letting the stream fail, we have another
> stream queued up that knows how to fetch the blocks elsewhere outside of
> our Hadoop cluster that couldn't be retrieved.  So we need to be able to
> catch the exception at this point, and these externally fetched bytes then
> get read into the user supplied read buffer.  Now Hadoop can proceed to
> read in the stream the next blocks in the file.
> So as you can see this method of fail over on demand allows an input
> stream to keep reading data, without having to start it all over again if a
> failure occurs (assuming the remote bytes were successfully fetched).
> As a final note I would like to mention that we will be providing our
> failover module to the Open Science Grid.  Since we hope to provide this as
> a benefit to all OSG users running at participating T2 computing clusters,
> we will be committed to maintaining this software and any changes to Hadoop
> needed to make it work.  In other words we will be willing to maintain any
> implementation changes that may become necessary as Hadoop internals change
> in future releases.
> Thanks,
> Jeff
> On 8/7/13 11:30 AM, Andrew Wang wrote:
>> I don't think exposing DFSClient and DistributedFileSystem members is
>> necessary to achieve what you're trying to do. We've got wrapper
>> FileSystems like FilterFileSystem and ViewFileSystem which you might be
>> able to use for inspiration, and the HCFS wiki lists some third-party
>> FileSystems that might also be helpful too.
>> On Wed, Aug 7, 2013 at 11:11 AM, Joe Bounour <jbounour@ddn.com> wrote:
>>  Hello Jeff
>>> Is it something that could go under HCFS project?
>>> http://wiki.apache.org/hadoop/**HCFS<http://wiki.apache.org/hadoop/HCFS>
>>> (I might be wrong?)
>>> Joe
>>> On 8/7/13 10:59 AM, "Jeff Dost" <jdost@ucsd.edu> wrote:
>>>  Hello,
>>>> We work in a software development team at the UCSD CMS Tier2 Center.  We
>>>> would like to propose a mechanism to allow one to subclass the
>>>> DFSInputStream in a clean way from an external package.  First I'd like
>>>> to give some motivation on why and then will proceed with the details.
>>>> We have a 3 Petabyte Hadoop cluster we maintain for the LHC experiment
>>>> at CERN.  There are other T2 centers worldwide that contain mirrors of
>>>> the same data we host.  We are working on an extension to Hadoop that,
>>>> on reading a file, if it is found that there are no available replicas
>>>> of a block, we use an external interface to retrieve this block of the
>>>> file from another data center.  The external interface is necessary
>>>> because not all T2 centers involved in CMS are running a Hadoop cluster
>>>> as their storage backend.
>>>> In order to implement this functionality, we need to subclass the
>>>> DFSInputStream and override the read method, so we can catch
>>>> IOExceptions that occur on client reads at the block level.
>>>> The basic steps required:
>>>> 1. Invent a new URI scheme for the customized "FileSystem" in
>>>> core-site.xml:
>>>>    <property>
>>>>      <name>fs.foofs.impl</name>
>>>>      <value>my.package.**FooFileSystem</value>
>>>>      <description>My Extended FileSystem for foofs: uris.</description>
>>>>    </property>
>>>> 2. Write new classes included in the external package that subclass the
>>>> following:
>>>> FooFileSystem subclasses DistributedFileSystem
>>>> FooFSClient subclasses DFSClient
>>>> FooFSInputStream subclasses DFSInputStream
>>>> Now any client commands that explicitly use the foofs:// scheme in paths
>>>> to access the hadoop cluster can open files with a customized
>>>> InputStream that extends functionality of the default hadoop client
>>>> DFSInputStream.  In order to make this happen for our use case, we had
>>>> to change some access modifiers in the DistributedFileSystem, DFSClient,
>>>> and DFSInputStream classes provided by Hadoop.  In addition, we had to
>>>> comment out the check in the namenode code that only allows for URI
>>>> schemes of the form "hdfs://".
>>>> Attached is a patch file we apply to hadoop.  Note that we derived this
>>>> patch by modding the Cloudera release hadoop-2.0.0-cdh4.1.1 which can be
>>>> found at:
>>>> http://archive.cloudera.com/**cdh4/cdh/4/hadoop-2.0.0-cdh4.**1.1.tar.gz<http://archive.cloudera.com/cdh4/cdh/4/hadoop-2.0.0-cdh4.1.1.tar.gz>
>>>> We would greatly appreciate any advise on whether or not this approach
>>>> sounds reasonable, and if you would consider accepting these
>>>> modifications into the official Hadoop code base.
>>>> Thank you,
>>>> Jeff, Alja & Matevz
>>>> UCSD Physics

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