Re: duplicate files and recent changes

[Hans Reiser <reiser@namesys.com>]

S. Alexander Jacobson wrote:

>On Thu, 6 Jun 2002, Robert Brockway wrote:
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>>>On Thu, Jun 06, 2002 at 01:30:46AM -0400, S. Alexander Jacobson wrote:
>>>      
>>>
>>>>I just joined this list.  Two question:
>>>>1. Can reiserfs detect that I have two copies of
>>>>the same file on disk and store tham as one file
>>>>        
>>>>
>>>Hm, you mean, each time you create a file, reiserfs should scan all
>>>other files and see if there is exactly a file like you just wrote?
>>>Hm, even something more complicated as you are writing to a file in
>>>4k chunks.
>>>Definitely no.
>>>      
>>>
>>And besides, how could it even know whether you even want them to be the
>>same or not. 
>>
windows uses a signature.

>> I keen plenty of identical files around (online backups,
>>etc) and I'd be mighty upset if the filesystem started hard linking them
>>together :)
>>    
>>
>
>That is why I said lazy copy on write. 
>
This is not patented, it is an old thread, everyone agrees that it 
should be done, no sponsor at the moment though.

> One
>implementation would be for the file system to
>keep a ref count of how many different files the
>user intended.  When an applicatiion writes to a
>file, the file system checks the ref count.  If
>the ref count is greater than 1, the file system
>copies the file, decrements the ref-count, and
>gives the writing application a pointer to the new
>copy.
>
>I'm sure there are smarter implementations, but
>the point is that the file system can certainly
>differentiate in theory between user hard links
>and actual hard links.
>
>-Alex-
>
>___________________________________________________________________
>S. Alexander Jacobson                   i2x Media
>1-212-787-1914 voice                    1-603-288-1280 fax
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