Function replace_start 

pub fn replace_start(
    fd: BorrowedFd<'_>,
    source: ReplaceSource<'_>,
    tgtdev_path: &CStr,
    avoid_srcdev: bool,
) -> Result<Result<(), ReplaceStartError>>

Start a device replace operation, copying all data from source to the target device at tgtdev_path.

When avoid_srcdev is true, the kernel will only read from the source device when no other zero-defect mirror is available (useful for replacing a device with known read errors).

Returns a two-level Result: the outer nix::Result covers ioctl-level failures (EPERM, EINVAL, etc.), while the inner Result covers application-level rejections reported by the kernel in the result field. Ok(Ok(())) means the replace started successfully. Ok(Err(AlreadyStarted)) means another replace is in progress. Ok(Err(ScrubInProgress)) means a scrub must finish or be cancelled first.

Errors: ENAMETOOLONG if source or target device paths exceed the kernel buffer size.