Re: [PATCH 0/2] btrfs: support cloned-device mount capability

[Anand Jain <anand.jain@oracle.com>]

On 10/6/23 15:42, Guilherme G. Piccoli wrote:
> Hi Anand / David, I was out at a conference and some holidays, so missed
> this patch. Is this a replacement of the temp-fsid approach?
> 
> So, to clarify a bit the inner workings of this patch: we don't have the
> temp-fsid superblock flag anymore? 

While btrfs doesn't need this superblock flag internally, we may
consider adding it for improved usability with other Btrfs features.

> Also, we can mount multiple
> partitions holding the same filesystem at the same time, given the
> nature of the patch (that generates the random fsid based on devt as per
> my superficial understanding) - right?

Indeed, devt remains unique to the partition we've utilized for a
similar purpose prior to this patch. Are there any devices lacking
a distinct devt value?


> And we don't use the
> metadata_uuid field here anymore,

Btrfs has always assigned fs_devices::metadata_uuid to either fsid
or metadata_uuid.


> i.e., we kinda "lose" the original fsid?

How? Have you tested to confirm?


> If that approaches is considered better than mine and works fine for the
> Steam Deck use case, 
> I'm glad in having that! 

As you have a use case to verify, can you indeed confirm whether
it works?

> But I would like at least
> to understand why it was preferred over the temp-fsid one, and what are
> the differences we can expect (need a flag to mkfs or can use btrfstune
> for that, for example).

The in-memory disk-super hack in the original patch is essentially a
workaround. This led to the necessity of restricting devices using
metadata_uuid from being used as temp-fsid device. A more appropriate
approach is to enhance device_list_add() to intelligently manage
duplicate disk-super entries by checking devt and permitting them
to mount if unique. This solution deviates from the original patch
and simultaneously addresses the subvol-mount corruption issue
observed in the original implementation.

The additional adjustments [1], such as sysfs interface, the constraints
on device additions, and the limitations on seed devices, are
supplementary patches essential to the comprehensive solution.

   [1] [PATCH 0/4] btrfs: sysfs and unsupported temp-fsid features for 
clones

However, the superblock temp-fsid flag isn't inherently necessary
within btrfs internals. Nevertheless, it can be considered for addition
if it makes the usability of other btrfs features with temp-fsid more 
seamless.

Thanks, Anand