Re: 'Subset' Hard Links

[Sunil Mushran <sunil.mushran@oracle.com>]

Micah Dombrowski wrote:
> I couldn't think of anywhere else to ask such a question, and google 
> is useless as I have no unique keywords.  I am wondering if it is 
> possible with some/any filesystems to have multiple hard links to a 
> file, some of which only point to a subset of the file's data.
>
> Eg:
> firstname -> all data bytes 1 to 10
> secondname -> bytes 3 to 10
> thirdname -> bytes 5 to 7
>
> This would clearly require some interesting locking of the file WRT 
> writes, but it seems like it should be possible, and even easy for 
> read-only access.  I deal with moderately large data files (50+GB), 
> and such a thing would be incredibly useful to me for pulling out 
> interesting bits of my data without having to make copies of the data 
> itself.
>
> A related method I was wishing existed would allow concatenation of 
> files simply by deleting all but one hard link, and changing the 
> remaining one to point to all of the original files' data as 
> fragments.  This would be great, as 'cat'ing together six 10GB files 
> is pretty slow, and it seems silly to be copying all of that data 
> around when I only need one actual instance of the full data on disk, 
> and that instance already exists, albeit in a fragmented manner.
>
> Do any tools for doing this sort of thing exist?
>

btrfs should able to handle most of this.

http://git.kernel.org/?p=linux/kernel/git/mason/btrfs-unstable.git;a=commitdiff;h=c5c9cd4d1b827fe545ed2a945e91e3a6909f3886

However, note that file systems operate in terms of blocks. So the start 
offset
would need to be block aligned.