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

On 10/14, Christoph Hellwig wrote:

On Mon, Oct 14, 2024 at 04:42:07PM +0000, Jaegeuk Kim wrote:

quoted

quoted

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.