Re: [PATCH] cowlinks v2

[Jamie Lokier <jamie@shareable.org>]

Pavel Machek wrote:
> Also it should be possible to have file with 2 hardlinks cowlinked
> somewhere, and possibly make more hardlinks of that one... Having
> pointer to another inode in place where direct block pointers normally
> are should be enough (thinking ext2 here).

Yes.

> > But sharing data blocks without sharing inodes is too horrible even to
> > contemplate, I suppose.
> 
> Why, btw?
> 
> Lets say we allocate 4 bits instead of one for block bitmap. Count
> "15" is special, now it means "15 or higher". That means we have to
> "garbage-collect" to free space that used to have more than 15 links,
> but that should not happen too often...

The garbage collection is what's horrible about it :)
Btw, 15 would be exceeded easily in my home directory.

IMHO, an inode whose block pointers points to another, so that whole
files only can be shared, would be fine.

Only one level of indirection would be allowed, so there'd be no
loops, just reference counted shared inodes.

-- Jamie

ps. Sharing individual data blocks is rather complicated.  If it were
a very desirable feature, I'd be inclined to go for reference counted
shared extents or reference counted shared indirection blocks
(i.e. sharing at the level or 4Mb or whatever the first indirection
block size is).