Re: COW files

[David Masover <ninja@slaphack.com>]

Gregory Maxwell wrote:

> Files could be COWed by a flag on the cp command (or really, perhaps
> that should be the default behavior) or with a utility (perhaps run as
> a periodic script to locate duplicates and COW them. This would
> greatly speed up the process of copying files.

Should be on the cp command, but you have to change the cp source for
that, and everything implementing "copy", including desktop managers.
There's currently no "copy" system call.  So it's more than just a plugin.

> The behavior on break could be to duplicate the whole cowed file on
> the first right, or allow a COWed file to have alternate choices for
> blocks. Files would remain cowed until decowed, which would likely be
> bad for performance (due to fragmentation of alternate versions
> causing gaps in sequential scans), so the repacker could be taught to
> decow files that have too many alternate blocks.

Not sure about this last one.  The repacker should increase the amount
of free space, not decrease it.  Especially, imagine a very large COWed
file, maybe a quarter the size of the FS, with 15 or so copies.  Yes,
it'll be fragmented, but the alternative is an FS full several times over.