Re: [LSF/MM/BPF TOPIC] User space RAID5 with ublk and io_uring BPF

[Ming Lei <ming.lei@redhat.com>]

Hi Xiao,

On Thu, Feb 19, 2026 at 01:38:46PM +0800, Xiao Ni wrote:
> Hi all
> 
> I'm doing some work on user-space RAID recently. I'd like to propose a
> topic for LSF/MM 2026 regarding the implementation of RAID5 in user
> space using ublk and io_uring bpf[1], particularly focusing on the
> challenges encountered and potential kernel improvements needed.
> 
> Ublk raid5 uses the ublk framework (tools/testing/selftests/ublk/),
> with the goal of leveraging io_uring's zero-copy capabilities and
> BPF[1] for performance optimization. The implementation includes:
> * RAID5 stripe handling with configurable chunk sizes
> * Multi-queue support via io_uring
> * Degraded mode for single disk failure tolerance
> * Integration with io_uring BPF struct_ops framework
> 
> During the implementation, I encountered several technical challenges.
> The primary challenge is performing XOR parity calculations in a true
> zero-copy manner:
> 
> 1. Bpf program support
> Raid5 needs to calculate parity with data. Now io_uring already
> supports zero copy. For targets such as raid0/raid1, it's enough. But
> for targets such as raid5, it still needs to copy data to userspace
> and calculate the parity. Bpf program is a nice solution[1] to resolve
> this issue. Besides the patch set[1], it still needs another bpf kfunc
> uring_bpf_xor() support.
> 
> 2. Register buffer per io
> Raid5 needs to calculate parity with data. So it still needs to
> pre-alloc buffer and register it for each io. The total memory is
> controlled by queue number and queue depth. With the pre-alloc memory
> and bpf program support, target such as raid5 can support true
> zero-copy manner.
> 
> Question for discussion:
> * Should the kernel provide XOR operation kfuncs for io_uring BPF?
> * What would be the appropriate API design?
> * Are there security or verification concerns with BPF performing XOR
> on registered buffers?
> 
> Current Status:
> * Basic RAID5 functionality: implemented
> * BPF struct_ops framework: successfully integrated
> * Performance testing: in progress
> * Main blocker: zero-copy XOR implementation waiting for kernel
> support I've written a patch for testing based on [1]
> 
> Why this matters
> User-space block devices (ublk) offer several advantages:
> * Faster development and iteration
> * Easier debugging
> *  Ability to leverage user-space libraries
> * No kernel panic risks during development
> * Easy to evaluate/compare performance between kernel raid and ublk
> based userspace raid.
> 
> Desired Outcome
> * If proposal in [1] would be acceptable
> * Get feedback on whether `uring_bpf_xor()` kfunc would be acceptable
> * Discuss API design for BPF-based computation on io_uring buffers
> * Understand the roadmap for io_uring + BPF capabilities
> * Learn best practices from the community for similar implementations

I am interested in this topic, and looks Caleb Sander Mateos has similar
requirement too.


Thanks,
Ming