Null Block Driver

The Rust null block driver rnull is an effort to implement a drop in replacement for null_blk in Rust.

A null block driver is a good opportunity to evaluate Rust bindings for the block layer. It is a small and simple driver and thus should be simple to reason about. Further, the null block driver is not usually deployed in production environments. Thus, it should be fairly straight forward to review, and any potential issues are not going to bring down any production workloads.

Being small and simple, the null block driver is a good place to introduce the Linux kernel storage community to Rust. This will help prepare the community for future Rust projects and facilitate a better maintenance process for these projects.

Statistics from the commit log of the C null_blk driver (before move) show that the C null block driver has had a significant amount of memory safety related problems in the past. 41% of fixes merged for the C null block driver are fixes for memory safety issues. This makes the null block driver a good candidate for rewriting in Rust.

The driver is implemented entirely in safe Rust, with all unsafe code fully contained in the abstractions that wrap the C APIs.

Features

Implemented features:

  • blk-mq support
  • Direct completion of IO
  • Read and write requests
  • Optional memory backing

Features available in the C null_blk driver that are currently not implemented in this work:

  • Bio-based submission
  • NUMA support
  • Block size configuration
  • Multiple devices
  • Dynamic device creation/destruction
  • Soft-IRQ and timer mode
  • Queue depth configuration
  • Queue count configuration
  • Discard operation support
  • Cache emulation
  • Bandwidth throttling
  • Per node hctx
  • IO scheduler configuration
  • Blocking submission mode
  • Shared tags configuration (for >1 device)
  • Zoned storage support
  • Bad block simulation
  • Poll queues

Resources

Performance

Setup

  • 12th Gen Intel(R) Core(TM) i5-12600
  • 32 GB DRAM
  • 1x INTEL MEMPEK1W016GA (PCIe 3.0 x2)
  • Debian Bullseye userspace

Results

In most cases there is less than 2% difference between the Rust and C drivers.

Contact

Please contact Andreas Hindborg through Zulip.