Re: [PATCH V3 0/2] Supporting same fsid mounting through the single-dev compat_ro feature

["Guilherme G. Piccoli" <gpiccoli@igalia.com>]

On 04/09/2023 03:36, Anand Jain wrote:
> [...]
> We need some manual tests as well to understand how this feature
> will behave in a multi-pathed device, especially in SAN and iSCSI
> environments.
> 
> I have noticed that Ubuntu has two different device
> paths for the root device during boot. It should be validated
> as the root file system using some popular OS distributions.
> 
> Thanks, Anand

Hi Anand, thanks for you suggestions! I just tried on Ubuntu 22.04.3 and
it worked flawlessly. After manually enabling the single-dev feature
through btrfstune, when booted into the distro kernel (6.2.x) it mounts
as RO (as expected, since this is a compat_ro feature). When switched to
a supporting kernel (6.5 + this patch), it mounts normally -
udev/systemd are capable of identifying the underlying device based on
UUID, and it mounts as SINGLE_DEV.

About the iSCSI / multipath cases, they are/should be unsupported. Is
multi-path a feature of btrfs? I think we should prevent users from
using single-dev along with other features of btrfs that doesn't make
sense, like we're doing here with devices removal/replace and of course,
with the metadata_uuid feature.

Now if multipath is not supported from btrfs, my understanding is that
users should not tune single-dev, as it doesn't make sense, but at the
same time it's not on scope here to test such scenario. In other words,
I'm happy to test a case that you suggest, but no matter how many
non-btrfs features/cases we test, we're not in control of what users can
do in the wild.

Cheers,


Guilherme