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Message-ID: <Zw6epcWDc13p1yCM@google.com>
Date: Tue, 15 Oct 2024 16:56:05 +0000
From: Jaegeuk Kim <jaegeuk@...nel.org>
To: Christoph Hellwig <hch@...radead.org>
Cc: Daeho Jeong <daeho43@...il.com>, Daeho Jeong <daehojeong@...gle.com>,
kernel-team@...roid.com, linux-kernel@...r.kernel.org,
linux-f2fs-devel@...ts.sourceforge.net,
linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org
Subject: 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:
> > >
> > > 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@...t_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@....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.
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