It is not possible to restore file from a mounted snapshot using a hardlink

It is not possible to restore file from a mounted snapshot using a hardlink

[dexen deVries]

Hello,


The following is more of a feature request than problem report, actually.

I've mounted an older snapshot and tried to restore (add it to the current 
filesystem state) by creating a hardlink.
Roughly like:
mount /dev/sdb3 ~/current
mount.nilfs2 /dev/sdb3 ~/old -o ro,cp=1234
cd ~/current
ln ~/old/file.bin ./restored-file.bin

The `ln' returned `Invalid cross-device link'.

I expected such hardlink to have semantics of re-using all the data blocks & 
metadata of file.bin from snapshot as restored-file.bin, so no duplication of 
data blocks happens.


Regards,
-- 
dexen deVries

``One can't proceed from the informal to the formal by formal means.''
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Re: It is not possible to restore file from a mounted snapshot using a hardlink

[Jérôme Poulin]

In fact, hard links are not supposed to work but cp --reflink
*should* work but it does not seem to. cp --reflink is what you would
need to achieve what you want.

Envoyé de mon appareil mobile.

Jérôme Poulin
Solutions G.A.

On 2011-02-27, at 08:20, dexen deVries <dexen.devries-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:

> Hello,
>
>
> The following is more of a feature request than problem report, actually.
>
> I've mounted an older snapshot and tried to restore (add it to the current
> filesystem state) by creating a hardlink.
> Roughly like:
> mount /dev/sdb3 ~/current
> mount.nilfs2 /dev/sdb3 ~/old -o ro,cp=1234
> cd ~/current
> ln ~/old/file.bin ./restored-file.bin
>
> The `ln' returned `Invalid cross-device link'.
>
> I expected such hardlink to have semantics of re-using all the data blocks &
> metadata of file.bin from snapshot as restored-file.bin, so no duplication of
> data blocks happens.
>
>
> Regards,
> --
> dexen deVries
>
> ``One can't proceed from the informal to the formal by formal means.''
> --
> To unsubscribe from this list: send the line "unsubscribe linux-nilfs" in
> the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
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Re: It is not possible to restore file from a mounted snapshot using a hardlink

[Jérôme Poulin]

Just after replying I noticed you talked about cp= and noticed I sent
to the nilfs mailing list and not btrfs !  So cp --reflink does not
work on NILFS but would be a nice feature :)

Envoyé de mon appareil mobile.

Jérôme Poulin
Solutions G.A.

On 2011-02-27, at 08:20, dexen deVries <dexen.devries-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:

> Hello,
>
>
> The following is more of a feature request than problem report, actually.
>
> I've mounted an older snapshot and tried to restore (add it to the current
> filesystem state) by creating a hardlink.
> Roughly like:
> mount /dev/sdb3 ~/current
> mount.nilfs2 /dev/sdb3 ~/old -o ro,cp=1234
> cd ~/current
> ln ~/old/file.bin ./restored-file.bin
>
> The `ln' returned `Invalid cross-device link'.
>
> I expected such hardlink to have semantics of re-using all the data blocks &
> metadata of file.bin from snapshot as restored-file.bin, so no duplication of
> data blocks happens.
>
>
> Regards,
> --
> dexen deVries
>
> ``One can't proceed from the informal to the formal by formal means.''
> --
> To unsubscribe from this list: send the line "unsubscribe linux-nilfs" in
> the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
--
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Re: It is not possible to restore file from a mounted snapshot using a hardlink

[dexen deVries]

On Sunday 27 of February 2011 16:56:25 you wrote:
> In fact, hard links are not supposed to work but cp --reflink
> *should* work but it does not seem to. cp --reflink is what you would
> need to achieve what you want.

I'm much surprised why hardlink shouldn't work.

I used the `stat' command on a file that's both in snapshot and (uchanged) in 
current filesystem state and both inode and device fields (st_ino and st_dev) 
are the same. Thus I don't get why Linux would consider that a cross-device 
hardlink. After all, it's the same filesystem, just different views of it. 
Perhaps Linux' VFS is to blame...


-- 
dexen deVries

``One can't proceed from the informal to the formal by formal means.''
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