Re: Atomic replacement of subvolumes is not possible

[Chris Mason <chris.mason@oracle.com>]

On Sun, Jun 27, 2010 at 07:44:12PM -0500, C Anthony Risinger wrote:
> On Sat, Jun 26, 2010 at 12:25 PM, Daniel Baumann <daniel@debian.org> =
wrote:
> > Hi,
> >
> > this is basically a forward from
> > http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=3D587253
> >
> > "rename(2) allows for the atomic replacement of files. =A0Being abl=
e to
> > atomically replace subvolume snapshots would be equally invaluable,
> > since it would permit lock-free replacement of subvolumes.
> >
> > =A0% btrfs subvolume snapshot <src> <dest>
> >
> > creates dest as a snapshot of src. However, if I want to do the
> > converse,
> >
> > =A0% btrfs subvolume snapshot <dest> <src>
> >
> > then <dest> is snapshotted as <src>/<dest>, i.e. not replacing the
> > original subvolume, but going inside the original subvolume.
> >
> > Use case 1:
> > =A0I have a subvolume of data under active use, which I want to
> > =A0periodically update. =A0I'd like to do this by atomically
> > =A0replacing its contents. =A0I can replace the content right now
> > =A0by deleting the old subvolume and then snapshotting the new
> > =A0on in its place, but it's racy. =A0It really needs to be
> > =A0replaced in a single operation, or else there's a small window
> > =A0where there is no data, and I'd need to resort to some external
> > =A0locking to protect myself.

I'm not sure I understand use case #1.  The problem is that you'll have
files open in the subvolume and you can't just pull the rug out from
under them.  Could you tell me a little more about what you're trying t=
o
do?

> >
> > Use case 2:
> > =A0In schroot, we create btrfs subvolume snapshots to get copy-on-
> > =A0write chroots. =A0This works just fine. =A0We also provide direc=
t
> > =A0access to the "source" subvolume, but since it could be
> > =A0snapshotted in an inconsistent state while being updated, we
> > =A0want to do the following:
> >
> > =A0=B7 snapshot source subvolume
> > =A0=B7 update snapshot
> > =A0=B7 replace source volume with updated snapshot"
> >
> > Please keep roger in the cc for any replies, thanks.
>=20
> i am also looking for functionality similar to this, except i would
> like to be able to replace the DEFAULT subvolume, with an empty or
> existing subvolume, and put the original default subvolume INSIDE the
> new root (or drop it completely), outlined by this post and the threa=
d
> it's in:
>=20
> http://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg05278.html
>=20
> is there any feedback on these actions?  no one seems to even respond=
 :-(
>=20
> it would seem we need ways to swap subvolumes around, _including_ the
> default, providing the on-disk format supports such operations.

Moving 'default' generally involves a reboot for the same reasons.  We
have to worry about open files and their view of the filesystem.  mv on
a directory won't affect file handles that are open, and renaming
subvolumes needs to follow a similar model.

-chris


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