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

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

[Sebastian Ochmann]

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

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

[Chris Murphy]

On Jul 16, 2014, at 4:18 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.

2+ computers writing to the same block device? I don't see how this is safe. Seems possibly a bug that the 1st mount event isn't setting some metadata so that another kernel instance knows not to allow another mount.


Chris Murphy

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

[Sebastian Ochmann]

Hello,

I need to clarify, I'm _not_ sharing a drive between multiple computers 
at the _same_ time. It's a portable device which I use at different 
locations with different computers. I just wanted to give a rationale 
for mounting the whole drive to some mountpoint and then also part of 
that drive (a subvolume) to the respective computer's /home mountpoint. 
So it's controlled by the same kernel in the same computer, it's just 
that part of the filesystem is mounted at multiple mountpoints, much 
like a bind-mount, but I'm interested in mounting a subvolume of the 
already-mounted volume to some other mountpoint. Sorry for the confusion.

Best regards
Sebastian


On 17.07.2014 01:18, Chris Murphy wrote:
>
> On Jul 16, 2014, at 4:18 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.
>
> 2+ computers writing to the same block device? I don't see how this is safe. Seems possibly a bug that the 1st mount event isn't setting some metadata so that another kernel instance knows not to allow another mount.
>
>
> Chris Murphy
>

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

[Qu Wenruo]

-------- Original Message --------
Subject: Re: Is it safe to mount subvolumes of already-mounted volumes 
(even with different options)?
From: Sebastian Ochmann <ochmann@informatik.uni-bonn.de>
To: Chris Murphy <lists@colorremedies.com>, zhe.zhang.research@gmail.com
Date: 2014年07月17日 15:58
> Hello,
>
> I need to clarify, I'm _not_ sharing a drive between multiple 
> computers at the _same_ time. It's a portable device which I use at 
> different locations with different computers. I just wanted to give a 
> rationale for mounting the whole drive to some mountpoint and then 
> also part of that drive (a subvolume) to the respective computer's 
> /home mountpoint. So it's controlled by the same kernel in the same 
> computer, it's just that part of the filesystem is mounted at multiple 
> mountpoints, much like a bind-mount, but I'm interested in mounting a 
> subvolume of the already-mounted volume to some other mountpoint. 
> Sorry for the confusion.
>
> Best regards
> Sebastian
If you mean something like the following use case:
# mount /dev/sdb1 -o subvolid=257 /home
# mount /dev/sdb1 -o subvolid=5 /some/other/place

That is completly OK.

But when it comes to different mount option, especially different ro/rw 
mount option,
although it is working for 3.16-rc*, the ro/rw mount option is still 
under disscussion and the current rc implement
will cause a kernel warning mounting a subvolume rw when it's first 
mounted as ro.

So in short:
1) mount subvolumes when the btrfs fs is already mounted.
Completly OK.

2) different mount option for different subvolume in one btrfs fs.
For most mount option including ro/rw, No.

Thanks,
Qu
>
>
> On 17.07.2014 01:18, Chris Murphy wrote:
>>
>> On Jul 16, 2014, at 4:18 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.
>>
>> 2+ computers writing to the same block device? I don't see how this 
>> is safe. Seems possibly a bug that the 1st mount event isn't setting 
>> some metadata so that another kernel instance knows not to allow 
>> another mount.
>>
>>
>> Chris Murphy
>>
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Re: Is it safe to mount subvolumes of already-mounted volumes (even with different options)?

[Zhe Zhang]

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
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

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

[Hugo Mills]

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

[Duncan]

Hugo Mills posted on Thu, 17 Jul 2014 09:41:53 +0100 as excerpted:

>> 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.

IIRC/AFAIK, the btrfs-specific mount options should be per filesystem, 
while stuff like relatime vs noatime is VFS level and should work per 
subvolume.

There's actually a current discussion about ro vs rw.  Consider the case 
of a parent subvolume (perhaps but not necessarily the root subvolume, 
id=5), being mounted writable in one location, with a child mounted 
elsewhere read-only.  Because it's possible to browse in the parent's 
subvolume down into the child subvolume as well, and someone could write 
a file there, that write would then show up in the elsewhere mounted read-
only child subvolume as well.

That's unexpected behavior to say the least!  Normally, read-only means 
it cannot and will not change, but in this case it wouldn't mean that at 
all!

My idea is that the same rules should apply to ro/rw as apply to btrfs 
snapshots -- they stop at subvolume borders.  Any write into a child 
subvolume would thus throw an error, regardless of how the parent 
subvolume was mounted.  The only way to write into a subvolume would be 
to mount it read-write on its own.  That would solve the ambiguity, but 
it would also be quite a change from existing behavior, where a read-
write mount of the root subvolume can write into any subvolume.

Someone else suggested that we separate filesystem read-write from 
subvolume read-write.  There's already the concept of read-only 
snapshots, used in btrfs-send, for one thing.  The idea here would be 
that a read-only filesystem/root mount means the entire filesystem is 
read-only, but provided the filesystem/root was mounted read-write, 
individual subvolumes could be mounted read-only using a different 
option, subv=ro, or similar, which would be hooked into the existing read-
only subvolume mechanism.  In that case, if the filesystem/root was read-
write, then the subvolume specific rw/ro mount option would take 
precedence and would trigger an error on write to that subvolume even if 
written from the read-write parent mount.

But while btrfs is the first filesystem to do this sort of thing and thus 
to deal with the problem, it might not be the last, so policy 
coordination with the VFS layer should be considered and a generic kernel 
policy for any filesystem dealing with subvolumes should be established.  
IOW, it's bigger than simply btrfs.

So anyway, while there was a patch applied earlier that did allow 
different read-only/read-write subvolume mounts, I believe that's 
reverted for 3.16, while this discussion continues and until it gets 
resolved one way or another, possibly at a kernel conference or the like.

But I believe generic VFS stuff like noatime/relatime/atime and dev/nodev/
suid/nosuid/exec/noexec is fine per-subvolume, because that's enforced at 
the VFS layer and there's no internal or expectation inconsistency to 
worry about if you can access for example the same device-file as a 
device via one mountpoint and not by another.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman