Module merge 

IO Merging

This module provides functionality to merge multiple sequential IO requests into a single larger request.

Overview

Merging IO requests can significantly improve performance by:

• Reducing the number of system calls (io_uring_enter or io_submit).
• Allowing larger sequential transfers which are often more efficient for storage devices.
• Reducing per-request overhead in the driver and completion handling.

Mechanism

The core component is IOMergeSubmitter, which buffers incoming IOEvents.

• Buffering: Events are added to EventMergeBuffer. They are merged if they are:

  • Sequential (contiguous offsets).
  • Same IO action (Read/Write).
  • Same file descriptor.
  • Total size does not exceed merge_size_limit.

• Flushing: When the buffer is full, the limit is reached, or flush() is called, the merged request is submitted.

• Sub-tasks:

  • If events are merged, a new “master” IOEvent is created covering the entire range.
  • The original events are attached as sub_tasks (a linked list) to this master event.
  • Write: The data from individual buffers is copied into a single large aligned buffer.
  • Read: A large buffer is allocated for the master event. Upon completion, data is copied back to the individual event buffers.
  • Completion: When the master event completes, callback_merged (in tasks.rs) is invoked. It iterates over sub-tasks, sets their results (copying data for reads), and triggers their individual callbacks.

Components

• EventMergeBuffer: Internal buffer logic.
• IOMergeSubmitter: Wraps a sender channel and manages the merge logic before sending.

Structs

EventMergeBuffer

IOMergeSubmitter

Try to merge sequential IOEvent.