On Thu, Jul 17, 2014 at 12:18:37AM +0200, Sebastian Ochmann wrote:
> 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.
You've said you're not sharing it concurrently, which is good -- as
long as you've only got one machine accessing it at the same time,
you're fine there.
> 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
Yes, absolutely:
hrm@amelia:~$ mount | grep btrfs
/dev/sda2 on /boot type btrfs (rw,noatime,space_cache)
/dev/sda2 on /home type btrfs (rw,noatime,space_cache)
/dev/sda2 on /media/video type btrfs (rw,noatime,space_cache)
/dev/sda2 on /media/pipeline type btrfs (rw,noatime,space_cache)
/dev/sda2 on /media/snarf type btrfs (rw,noatime,space_cache)
/dev/sda2 on /media/audio type btrfs (rw,noatime,space_cache)
/dev/sda2 on /srv/nfs/home type btrfs (rw,noatime,space_cache)
/dev/sda2 on /srv/nfs/video type btrfs (rw,noatime,space_cache)
/dev/sda2 on /srv/nfs/testing type btrfs (rw,noatime,space_cache)
/dev/sda2 on /srv/nfs/pipeline type btrfs (rw,noatime,space_cache)
/dev/sda2 on /srv/nfs/audio type btrfs (rw,noatime,space_cache)
/dev/sda2 on /srv/nfs/nadja type btrfs (rw,noatime,space_cache)
> 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?
If you assume that the first mount options are the ones used for
everything, regardless of any different options provided in subsequent
mounts, then you probably won't go far wrong. It's not quite true:
some options do work on a per-mount basis, but most are
per-filesystem. I'm sure there was a list of them on the wiki at some
point, but I can't seem to track it down right now.
Hugo.
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