Re: [gentoo-dev] GSoC proposal: cp --reflink support for zfs.

[Yuxuan Shui]

Hi,

On Wed, Mar 12, 2014 at 9:30 PM, Richard Yao <ryao@g.o> wrote:
>
> On 03/12/2014 08:45 AM, Alex Xu wrote:
> > On 12/03/14 03:15 AM, Yuxuan Shui wrote:
> >> Hi,
> >>
> >> I would like to implement cp --reflink support for ZFSOnLinux as my GSoC
> >> project.
> >>
> >> cp --reflink is used to create a COW copy of a file, so the file will not
> >> take any disk space if it's not modified. This feature is very useful for
> >> cases like storing a lot of almost identical virtual machine images. Also
> >> this is a frequently requested feature for ZoL. [1][2][3]
> >>
> >> Currently only btrfs support this feature, so my goal it to bring it to ZoL
> >> as well.
> >>
> >> I think the only way to do it (without changing too many parts of ZoL) is
> >> to use the deduplication feature of zfs. A COW copy could be done by create
> >> a new entry in ddt for the old file, and create a new file which points to
> >> the ddt entry.
> >>
> >> Please let me know if this proposal makes sense, and if that's the right
> >> way to do it.
> >>
> >> Thanks.
> >>
> >> [1]:
> >> https://groups.google.com/a/zfsonlinux.org/forum/#!topic/zfs-discuss/mvGB7QEpt3w
> >> [2]: https://github.com/zfsonlinux/zfs/issues/405
> >> [3]: https://github.com/zfsonlinux/zfs/issues/1063
> >>
> >
> > While I can't comment too much on the technical aspects, they seem to be
> > relatively sound.
> >
> > However, there are some issues with the, er... other aspects, for lack
> > of better terminology.
> >
> > 1. This is possibly out of scope as a Gentoo project, since ZOL is not
> > really part of Gentoo. If it's not, then you're out of luck, because ZOL
> > is not an accepted organization.
>
> Things that provide us with improvements over what we have are
> definitely worth consideration as GSoC projects. However, what is
> accepted ultimately depends on not only feedback from a potential
> mentor, but also a vote of Gentoo developers.

Maybe I could propose this project to illumos? Since they seems to
accept proposal for OpenZFS.

>
> > 2. This is likely too small to be a GSoC project. Perhaps see [0] for a
> > list of example ideas, if only so you can get a grasp on the size of a
> > good project.
> >
> > It does sound like a good idea though, and even if you can't do it as
> > part of GSoC, you should pursue it anyways.
>
> Leaning on my understanding of ZFS internals, I can say that this is
> large enough. However, I do not think it will accomplish the desired
> result if implemented in the manner suggested.

Could you elaborate why this won't work? Thanks.

>

-- 

Regards
Yuxuan Shui