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From Arpit Agarwal <aagar...@hortonworks.com>
Subject Re: Replace a block with a new one
Date Fri, 18 Jul 2014 21:18:58 GMT
IMHO this is a spectacularly bad idea. Is it a one off event? Why not just
take the perf hit and recreate the file?

If you need to do this regularly you should consider a mutable file store
like HBase. If you start modifying blocks from under HDFS you open up all
sorts of consistency issues.




On Fri, Jul 18, 2014 at 2:10 PM, Shumin Guo <gsmsteve@gmail.com> wrote:

> That will break the consistency of the file system, but it doesn't hurt to
> try.
> On Jul 17, 2014 8:48 PM, "Zesheng Wu" <wuzesheng86@gmail.com> wrote:
>
>> How about write a new block with new checksum file, and replace the old
>> block file and checksum file both?
>>
>>
>> 2014-07-17 19:34 GMT+08:00 Wellington Chevreuil <
>> wellington.chevreuil@gmail.com>:
>>
>>> Hi,
>>>
>>> there's no way to do that, as HDFS does not provide file updates
>>> features. You'll need to write a new file with the changes.
>>>
>>> Notice that even if you manage to find the physical block replica files
>>> on the disk, corresponding to the part of the file you want to change, you
>>> can't simply update it manually, as this would give a different checksum,
>>> making HDFS mark such blocks as corrupt.
>>>
>>> Regards,
>>> Wellington.
>>>
>>>
>>>
>>> On 17 Jul 2014, at 10:50, Zesheng Wu <wuzesheng86@gmail.com> wrote:
>>>
>>> > Hi guys,
>>> >
>>> > I recently encounter a scenario which needs to replace an exist block
>>> with a newly written block
>>> > The most straightforward way to finish may be like this:
>>> > Suppose the original file is A, and we write a new file B which is
>>> composed by the new data blocks, then we merge A and B to C which is the
>>> file we wanted
>>> > The obvious shortcoming of this method is wasting of network bandwidth
>>> >
>>> > I'm wondering whether there is a way to replace the old block by the
>>> new block directly.
>>> > Any thoughts?
>>> >
>>> > --
>>> > Best Wishes!
>>> >
>>> > Yours, Zesheng
>>>
>>>
>>
>>
>> --
>> Best Wishes!
>>
>> Yours, Zesheng
>>
>

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