I'd like to discuss an interface to implement user space block devices, while avoiding local network NBD solutions. There has been reiterated interest in the topic, both from researchers [1] and from the community, including a proposed session in LSFMM2018 [2] (though I don't think it happened). I've been working on top of the Google iblock implementation to find something upstreamable and would like to present my design and gather feedback on some points, in particular zero-copy and overall user space interface. The design I'm pending towards uses special fds opened by the driver to transfer data to/from the block driver, preferably through direct splicing as much as possible, to keep data only in kernel space. This is because, in my use case, the driver usually only manipulates metadata, while data is forwarded directly through the network, or similar. It would be neat if we can leverage the existing splice/copy_file_range syscalls such that we don't ever need to bring disk data to user space, if we can avoid it. I've also experimented with regular pipes, But I found no way around keeping a lot of pipes opened, one for each possible command 'slot'. [1] https://dl.acm.org/doi/10.1145/3456727.3463768 [2] https://www.spinics.net/lists/linux-fsdevel/msg120674.html -- Gabriel Krisman Bertazi
