Re: [f2fs-dev] [PATCH v5] f2fs: introduce device aliasing file

[Jaegeuk Kim <jaegeuk@kernel.org>]

On 10/14, Christoph Hellwig wrote:
> On Mon, Oct 14, 2024 at 04:42:07PM +0000, Jaegeuk Kim wrote:
> > > 
> > > Plz, refer to this patch and the description there.
> > > 
> > > https://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git/commit/?h=dev-test&id=8cc4e257ec20bee207bb034d5ac406e1ab31eaea
> > 
> > Also, I added this in the description.
> > 
> > ---
> >     For example,
> >     "mkfs.f2fs -c /dev/block/test@test_alias /dev/block/main" gives
> >     a file $root/test_alias which carves out /dev/block/test partition.
> 
> What partition?
> 
> So mkfs.f2fs adds additional devices based on the man page.
> 
> So the above creates a file system with two devices, but the second
> device is not added to the general space pool, but mapped to a specific
> file?  How does this file work.  I guess it can't be unlinked and
> renamed.  It probably also can't be truncated and hole punched,
> or use insert/collapse range.  How does the user find out about this
> magic file?  What is the use case?  Are the exact semantics documented
> somewhere?

Let me ask for putting some design in Documentation. Just for a quick reference,
the use-case looks like:

# ls /dev/vd*
/dev/vdb (32GB) /dev/vdc (32GB)
# mkfs.ext4 /dev/vdc
# mkfs.f2fs -c /dev/vdc@vdc.file /dev/vdb
# mount /dev/vdb /mnt/f2fs
# ls -l /mnt/f2fs
vdc.file
# df -h
/dev/vdb                            64G   33G   32G  52% /mnt/f2fs

# mount -o loop /dev/vdc /mnt/ext4
# df -h
/dev/vdb                            64G   33G   32G  52% /mnt/f2fs
/dev/loop7                          32G   24K   30G   1% /mnt/ext4
# umount /mnt/ext4

# f2fs_io getflags /mnt/f2fs/vdc.file 
get a flag on /mnt/f2fs/vdc.file ret=0, flags=nocow(pinned),immutable
# f2fs_io setflags noimmutable /mnt/f2fs/vdc.file
get a flag on noimmutable ret=0, flags=800010
set a flag on /mnt/f2fs/vdc.file ret=0, flags=noimmutable
# rm /mnt/f2fs/vdc.file
# df -h
/dev/vdb                            64G  753M   64G   2% /mnt/f2fs

So, key idea is, user can do any file operations on /dev/vdc, and
reclaim the space after the use, while the space is counted as /data.
That doesn't require modifying partition size and filesystem format.