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From Shumin Guo <gsmst...@gmail.com>
Subject Re: basic question about rack awareness and computation migration
Date Thu, 07 Mar 2013 16:05:15 GMT
Yes, I agree with Bertrand. Hadoop can take a whole file as input and you
just put your compression code into the map method, and use the identity
reduce function that simply writes your compressed data on to HDFS by using
the file output format.


On Thu, Mar 7, 2013 at 7:35 AM, Bertrand Dechoux <dechouxb@gmail.com> wrote:

> I might have missed something but is there a reason for the input of the
> mappers to be a list of files and not the files themselves?
> The usual way is to provide a path to the files that should be processed
> and then Hadoop will figure for you how to best use data locality.
> Is there a reason for not doing that?
> How big is each image file? How are they stored?
> You could create an input format not splittable (it is a simple property),
> that way you are sure that a mapper will process the whole file.
> And then trivially your mapper compresses the provided image, Hadoop will
> use a mapper per file and deals with data locality by itself.
> Regards
> Bertrand
> On Wed, Mar 6, 2013 at 4:43 AM, Julian Bui <julianbui@gmail.com> wrote:
>> Thanks Harsh,
>> > Are your input lists big (for each compressed output)? And is the list
>> arbitrary or a defined list per goal?
>> I dictate what my inputs will look like.  If they need to be list of
>> image files, then I can do that.  If they need to be the images themselves
>> as you suggest, then I can do that too but I'm not exactly sure what that
>> would look like.  Basically, I will try to format my inputs in the way that
>> makes the most sense from a locality point of view.
>> Since all the keys must be writable, I explored the Writable interface
>> and found the interesting sub-classes:
>>    - FileSplit
>>    - BlockLocation
>>    - BytesWritable
>> These all look somewhat promising as they kind of reveal the location
>> information of the files.
>> I'm not exactly sure how I would use these to hint at the data locations.
>>  Since these chunks of the file appear to be somewhat arbitrary in size and
>> offset, I don't know how I could perform imagery operations on them.  For
>> example, if I knew that bytes 0x100-0x400 lie on node X, then that makes it
>> difficult for me to use that information to give to my image libraries -
>> does 0x100-0x400 correspond to some region/MBR within the image?  I'm not
>> sure how to make use of this information.
>> The responses I've gotten so far indicate to me that HDFS kind of does
>> the computation migration for me but that I have to give it enough
>> information to work with.  If someone could point to some detailed reading
>> about this subject that would be pretty helpful, as I just can't find the
>> documentation for it.
>> Thanks again,
>> -Julian
>> On Tue, Mar 5, 2013 at 5:39 PM, Harsh J <harsh@cloudera.com> wrote:
>>> Your concern is correct: If your input is a list of files, rather than
>>> the files themselves, then the tasks would not be data-local - since
>>> the task input would just be the list of files, and the files' data
>>> may reside on any node/rack of the cluster.
>>> However, your job will still run as the HDFS reads do remote reads
>>> transparently without developer intervention and all will still work
>>> as you've written it to. If a block is found local to the DN, it is
>>> read locally as well - all of this is automatic.
>>> Are your input lists big (for each compressed output)? And is the list
>>> arbitrary or a defined list per goal?
>>> On Tue, Mar 5, 2013 at 5:19 PM, Julian Bui <julianbui@gmail.com> wrote:
>>> > Hi hadoop users,
>>> >
>>> > I'm trying to find out if computation migration is something the
>>> developer
>>> > needs to worry about or if it's supposed to be hidden.
>>> >
>>> > I would like to use hadoop to take in a list of image paths in the
>>> hdfs and
>>> > then have each task compress these large, raw images into something
>>> much
>>> > smaller - say jpeg  files.
>>> >
>>> > Input: list of paths
>>> > Output: compressed jpeg
>>> >
>>> > Since I don't really need a reduce task (I'm more using hadoop for its
>>> > reliability and orchestration aspects), my mapper ought to just take
>>> the
>>> > list of image paths and then work on them.  As I understand it, each
>>> image
>>> > will likely be on multiple data nodes.
>>> >
>>> > My question is how will each mapper task "migrate the computation" to
>>> the
>>> > data nodes?  I recall reading that the namenode is supposed to deal
>>> with
>>> > this.  Is it hidden from the developer?  Or as the developer, do I
>>> need to
>>> > discover where the data lies and then migrate the task to that node?
>>>  Since
>>> > my input is just a list of paths, it seems like the namenode couldn't
>>> really
>>> > do this for me.
>>> >
>>> > Another question: Where can I find out more about this?  I've looked up
>>> > "rack awareness" and "computation migration" but haven't really found
>>> much
>>> > code relating to either one - leading me to believe I'm not supposed
>>> to have
>>> > to write code to deal with this.
>>> >
>>> > Anyway, could someone please help me out or set me straight on this?
>>> >
>>> > Thanks,
>>> > -Julian
>>> --
>>> Harsh J

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