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

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Message-ID: <Zw34CMxJB-THlGW0@infradead.org>
Date: Mon, 14 Oct 2024 22:05:12 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Jaegeuk Kim <jaegeuk@...nel.org>
Cc: Daeho Jeong <daeho43@...il.com>, Christoph Hellwig <hch@...radead.org>,
	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 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?