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by loeg 1404 days ago
> Block devices operate on blocks of data identified by offset.

Sure, although CUSE read and write operations take offsets, too. The kernel could just send block-sized IOs to a CUSE driver and it wouldn't be all that different.

> You can in principle implement a block device-ish API in FUSE by disabling open/close and requiring all reads/writes to be at given offsets

Right, ok.

I think the historical distinction between block and character devices is largely that -- historical. Nowadays the distinction is mostly whether or not the kernel puts a block cache in front of the device. FreeBSD eliminated the distinction entirely.
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jmillikin 1404 days ago
There may be kernels that have simplified their device model to unify character and block devices, but Linux has not. FUSE/CUSE (and now ublk) are Linux-oriented protocols from the beginning, with relatively little thought given to cross-platform compatibility.

If you use FreeBSD then you're likely familiar with the challenges they've faced adapting FUSE to their VFS, and last time I checked they don't have plans to support CUSE at all.

You might also be interested in <https://lwn.net/Articles/343514/> (from 2009!), which discusses some of the challenges with using something like the FUSE protocol to back a block device in Linux. That message also describes a better solution which, to my eyes, looks a lot like ublk.

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loeg 1404 days ago
FreeBSD does have CUSE, for what it’s worth.
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jmillikin 1404 days ago
FreeBSD has a /dev/cuse device, and a libcuse reimplemented on top of it, but it uses a different protocol from Linux's CUSE. You can see the FreeBSD implementation at https://github.com/freebsd/freebsd-src/blob/release/13.1.0/s... -- note how cuse_server_read() and cuse_server_write() are stubs.

I am somewhat familiar with this because I wrote a FUSE/CUSE server library in Rust, and tried porting it to FreeBSD. The FUSE bits worked with only minor issues[0][1], but the CUSE bits were completely different so I had to turn off that part of the library for FreeBSD targets.

[0] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=253411

[1] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=253500

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