Re: Is it safe to mount subvolumes of already-mounted volumes (even with different options)?

[Zhe Zhang <zhe.zhang.research@gmail.com>]

Hi Sebastian,

I posted a similar question and got many helpful answers:
https://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg35047.html

Basically, you cannot guarantee that the computing mounting /dev/sdx
doesn't write to arbitrary addresses of /dev/sdxN as unallocated
blocks and thus corrupting the file system on it (I assume sdxN in
your example is a sub volume of sdx).

On Wed, Jul 16, 2014 at 6:18 PM, Sebastian Ochmann
<ochmann@informatik.uni-bonn.de> wrote:
> Hello,
>
> I'm sharing a btrfs-formatted drive between multiple computers and each of
> the machines has a separate home directory on that drive. The root of the
> drive is mounted at /mnt/tray and the home directory for machine {hostname}
> is under /mnt/tray/Homes/{hostname}. Up until now, I have mounted /mnt/tray
> like a normal volume and then did an additional bind-mount of
> /mnt/tray/Homes/{hostname} to /home.
>
> Now I have a new drive and wanted to do things a bit more advanced by
> creating subvolumes for each of the machines' home directories so that I can
> also do independent snapshotting. I guess I could use the bind-mount method
> like before but my question is if it is considered safe to do an additional,
> "regular" mount of one of the subvolumes to /home instead, like
>
> mount /dev/sdxN /mnt/tray
> mount -o subvol=/Homes/{hostname} /dev/sdxN /home
>
> When I experimented with such additional mounts of subvolumes of
> already-mounted volumes, I noticed that the mount options of the additional
> subvolume mount might differ from the "original" mount. For instance, the
> root volume might be mounted with "noatime" while the subvolume mount may
> have "relatime".
>
> So my questions are: Is mounting a subvolume of an already mounted volume
> considered safe and are there any combinations of possibly conflicting mount
> options one should be aware of (compression, autodefrag, cache clearing)? Is
> it advisable to use the same mount options for all mounts pointing to the
> same physical device?
>
> Best regards,
> Sebastian
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