Deep Dive: Tracing the `write()` System Call in OpenBSD
2025-03-29
This article delves into the low-level implementation of the `write()` system call in OpenBSD. Starting from the user-space `write()` call, it traces the data's journey through the kernel, detailing the complete path from system call to data written to an NVMe hard drive. The article reveals a chain of kernel function calls, including `mi_syscall`, `sys_write`, `dofilewritev`, `vn_write`, `ffs_write`, `uiomove`, `bdwrite`, `syncer`, `bwrite`, `ufs_strategy`, `spec_strategy`, `sdstrategy`, and finally the NVMe driver. It illustrates data transformation and transmission across different abstraction layers, highlighting key technical details such as caching mechanisms and DMA transfers.
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Development
System Call