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

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

   Gentle ping on any comments

Thanks, Anand


On 28/09/2023 09:09, Anand Jain wrote:
> Guilherme's previous work [1] aimed at the mounting of cloned devices
> using a superblock flag SINGLE_DEV during mkfs.
>   [1] https://lore.kernel.org/linux-btrfs/20230831001544.3379273-1-gpiccoli@igalia.com/
> 
> Building upon this work, here is in-memory only approach. As it mounts
> we determine if the same fsid is already mounted if then we generate a
> random temp fsid which shall be used the mount, in-memory only not
> written to the disk. We distinguish device by devt.
> 
> Mount option / superblock flag:
> -------------------------------
>   These patches show we don't have to limit the single-device / temp_fsid
> capability with a mount option or a superblock flag from the btrfs
> internals pov. However, if necessary from the user's perspective,
> we can add them later on top of this patch. I've prepared a mount option
> -o temp_fsid patch, but I'm not included at this time. As most of the
> tests was without it.
> 
> Compatible with other features that may be affected:
> ----------------------------------------------------
>   Multi device:
>      A btrfs filesytem on a single device can be copied using dd and
>      mounted simlutaneously. However, a multi device btrfs copied using
>      dd and trying to mount simlutaneously is forced to fail:
> 
>        mount: /btrfs1: mount(2) system call failed: File exists.
> 
>   Send and receive:
>      Quick tests shows send and receive between two single devices with
>      the same fsid mounted on the _same_ host works!.
>      (Also, the receive-mnt can receive from multiple senders as long as
>      conflits are managed externally. ;-).)
> 
>   Replace:
>       Works fine.
> 
> btrfs-progs:
> ------------
>   btrfs-progs needs to be updated to support the commands such as
> 
> 	btrfs filesystem show
> 
>   when devices are not mounted. So the device list is not based on
>   the fisd any more.
> 
> Testing:
> -------
>   This patch has been under testing for some time. The challenge is to get
>   the fstests to test this reasonably well.
> 
>   As of now, this patch runs fine on a large set of fstests test cases
>   using a custom-built mkfs.btrfs with the -U option and a new -P option
>   to copy the device FSID and UUID from the TEST_DEV to the SCRATCH_DEV
>   at the scratch_mkfs time. For example:
> 
>    Config file:
> 
>       config_fsid=$(btrfs in dump-super $TEST_DEV | grep -E ^fsid | \
> 							awk '{print $2}')
>       config_uuid=$(btrfs in dump-super $TEST_DEV | \
> 				grep -E ^dev_item.uuid | awk '{print $2}')
>       MKFS_OPTIONS="-U $config_fsid -P $config_uuid"
> 
>   This configuration option ensures that both TEST_DEV and SCRATCH_DEV will
>   have the same FSID and device UUID while still applying test-specific
>   scratch mkfs options.
> 
> Mkfs.btrfs:
> -----------
>   mkfs.btrfs needs to be updated to support the -P option for testing only.
> 
>     btrfs-progs: allow duplicate fsid for single device
>     btrfs-progs: add mkfs -P option for dev_uuid
> 
> Anand Jain (2):
>    btrfs: add helper function find_fsid_by_disk
>    btrfs: support cloned-device mount capability
> 
>   fs/btrfs/disk-io.c |  3 +-
>   fs/btrfs/volumes.c | 75 +++++++++++++++++++++++++++++++++++++++++++---
>   fs/btrfs/volumes.h |  2 ++
>   3 files changed, 75 insertions(+), 5 deletions(-)
>