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@xxxxxxxx /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.
