diff --git a/src/uu/sed/src/fast_io.rs b/src/uu/sed/src/fast_io.rs
index 514f493..b70e453 100644
--- a/src/uu/sed/src/fast_io.rs
+++ b/src/uu/sed/src/fast_io.rs
@@ -1,7 +1,7 @@
 // Zero-copy line-based I/O
 //
 // Abstractions that allow file lines to be processed and output
-// in mmapped memory space.  By coallescing output requests an
+// in mmapped memory space.  By coalescing output requests an
 // efficient write(2) system call can be issued for them, bypassing
 // the copy required for output through BufWriter.
 // Search for "main" to see a usage example.
@@ -26,29 +26,34 @@ use std::io::{self, BufRead, BufReader, BufWriter, Read, Write};
 #[cfg(not(unix))]
 use std::marker::PhantomData;
 
+#[cfg(unix)]
+use std::os::fd::RawFd;
+
 #[cfg(unix)]
 use std::os::unix::io::AsRawFd;
 
-use std::path::PathBuf;
 use std::str;
 
-#[cfg(unix)]
-use uucore::libc::{c_void, write};
-
+use std::path::PathBuf;
 use uucore::error::UError;
 
 #[cfg(unix)]
 use uucore::error::USimpleError;
 
+#[cfg(unix)]
+use uucore::libc;
+
 // Define two cursors for iterating over lines:
 // - MmapLineCursor based on mmap(2),
-// - ReadLineCursorbased on BufReader.
+// - ReadLineCursor based on BufReader.
 
 /// Cursor for zero-copy iteration over mmap’d file.
 #[cfg(unix)]
 pub struct MmapLineCursor<'a> {
-    data: &'a [u8],
-    pos: usize,
+    _file: File,         // Mmapped file; keeps file open for fast copy
+    fast_copy: FastCopy, // Data for fast file copy I/O
+    data: &'a [u8],      // Mmapped data
+    pos: usize,          // Position within the data
 }
 
 #[cfg(unix)]
@@ -61,8 +66,13 @@ pub struct NextMmapLine<'a> {
 
 #[cfg(unix)]
 impl<'a> MmapLineCursor<'a> {
-    fn new(data: &'a [u8]) -> Self {
-        Self { data, pos: 0 }
+    fn new(file: File, data: &'a [u8]) -> Self {
+        Self {
+            fast_copy: FastCopy::new(&file),
+            _file: file,
+            data,
+            pos: 0,
+        }
     }
 
     /// Return the next line, if available, or None.
@@ -254,18 +264,17 @@ impl<'a> IOChunk<'a> {
         match &self.content {
             IOChunkContent::Owned { .. } => Ok(()), // already owned
             #[cfg(unix)]
-            IOChunkContent::MmapInput { content, full_span } => {
-                match std::str::from_utf8(content) {
-                    Ok(valid_str) => {
-                        let has_newline = full_span.last().copied() == Some(b'\n');
-                        self.content =
-                            IOChunkContent::new_owned(valid_str.to_string(), has_newline);
-                        self.utf8_verified.set(true);
-                        Ok(())
-                    }
-                    Err(e) => Err(USimpleError::new(2, e.to_string())),
+            IOChunkContent::MmapInput {
+                content, full_span, ..
+            } => match std::str::from_utf8(content) {
+                Ok(valid_str) => {
+                    let has_newline = full_span.last().copied() == Some(b'\n');
+                    self.content = IOChunkContent::new_owned(valid_str.to_string(), has_newline);
+                    self.utf8_verified.set(true);
+                    Ok(())
                 }
-            }
+                Err(e) => Err(USimpleError::new(2, e.to_string())),
+            },
         }
     }
 
@@ -286,13 +295,15 @@ impl<'a> IOChunk<'a> {
 }
 
 /// Data to be written to a file. It can come from the mmapped
-/// memory space, in which case it is tracked to allow coallescing
+/// memory space, in which case it is tracked to allow coalescing
 /// and bypassing BufWriter, or it can be other data from the process's
 /// memory space.
 #[derive(Debug, PartialEq, Eq)]
 enum IOChunkContent<'a> {
     #[cfg(unix)]
     MmapInput {
+        fast_copy: FastCopy, // Data for fast file copy I/O
+        base: *const u8,     // Mmap start address
         content: &'a [u8],   // Line without newline
         full_span: &'a [u8], // Line including original newline, if any
     },
@@ -343,24 +354,74 @@ impl IOChunkContent<'_> {
     }
 }
 
+/// Information required for performing I/O using fast file copy
+/// operations, such as copy_file_range(2).
+#[cfg(unix)]
+#[derive(Debug, PartialEq, Eq, Clone)]
+struct FastCopy {
+    fd: i32,           // Raw file descriptor
+    is_regular: bool,  // True if this is a regular file
+    block_size: usize, // Filesystem block size
+}
+
+#[cfg(unix)]
+impl FastCopy {
+    /// Construct with an object on which as_raw_fd() can be called.
+    pub fn new<T: AsRawFd + ?Sized>(f: &T) -> Self {
+        let fd = f.as_raw_fd();
+
+        let mut st: libc::stat = unsafe { std::mem::zeroed() };
+
+        let ret = unsafe { libc::fstat(fd, &mut st) };
+        if ret == -1 {
+            // All fstat errors are programmer rather user faults
+            // so panic is appropriate.
+            let err = std::io::Error::last_os_error();
+            panic!("fstat failed on fd {}: {}", fd, err);
+        }
+
+        let ftype = st.st_mode & libc::S_IFMT;
+
+        Self {
+            fd,
+            is_regular: ftype == libc::S_IFREG,
+            block_size: st.st_blksize as usize,
+        }
+    }
+}
+
+#[cfg(test)]
+#[cfg(unix)]
+impl Default for FastCopy {
+    fn default() -> Self {
+        FastCopy {
+            fd: -1,
+            is_regular: false,
+            block_size: 0,
+        }
+    }
+}
+
 /// Unified reader that uses mmap when possible, falls back to buffered reading.
-pub enum LineReader {
+pub enum LineReader<'a> {
     #[cfg(unix)]
     MmapInput {
         mapped_file: Mmap, // A handle that can derive the mapped file slice
-        cursor: MmapLineCursor<'static>,
+        cursor: MmapLineCursor<'a>,
     },
     ReadInput(ReadLineCursor),
+    #[cfg(not(unix))]
+    _Phantom(std::marker::PhantomData<&'a ()>),
 }
 
 /// Return a LineReader that uses the ReadInput method fot the specified file.
-fn line_reader_read_input(file: File) -> io::Result<LineReader> {
+fn line_reader_read_input(file: File) -> io::Result<LineReader<'static>> {
     let boxed: Box<dyn Read> = Box::new(file);
     let reader = BufReader::new(boxed);
     Ok(LineReader::ReadInput(ReadLineCursor::new(reader)))
 }
 
-impl LineReader {
+impl<'a> LineReader<'a> {
     /// Open the specified file for line input.
     // Use "-" to read from the standard input.
     pub fn open(path: &PathBuf) -> io::Result<Self> {
@@ -381,7 +442,7 @@ impl LineReader {
                     let slice: &'static [u8] = unsafe {
                         std::slice::from_raw_parts(mapped_file.as_ptr(), mapped_file.len())
                     };
-                    let cursor = MmapLineCursor::new(slice);
+                    let cursor = MmapLineCursor::new(file, slice);
                     Ok(LineReader::MmapInput {
                         mapped_file,
                         cursor,
@@ -411,14 +472,21 @@ impl LineReader {
         match self {
             #[cfg(unix)]
             LineReader::MmapInput { cursor, .. } => {
+                // Obtain fields to prevent borrowing issues.
+                let fast_copy = cursor.fast_copy.clone();
+                let base = cursor.data.as_ptr();
                 if let Some(NextMmapLine {
                     content,
                     full_span,
                     is_last_line,
                 }) = cursor.get_line()?
                 {
-                    let chunk =
-                        IOChunk::from_content(IOChunkContent::MmapInput { content, full_span });
+                    let chunk = IOChunk::from_content(IOChunkContent::MmapInput {
+                        fast_copy,
+                        base,
+                        content,
+                        full_span,
+                    });
 
                     Ok(Some((chunk, is_last_line)))
                 } else {
@@ -435,6 +503,9 @@ impl LineReader {
                     Ok(None)
                 }
             }
+
+            #[cfg(not(unix))]
+            LineReader::_Phantom(_) => unreachable!("_Phantom should never be constructed"),
         }
     }
 }
@@ -450,27 +521,35 @@ pub trait OutputWrite: Write {}
 #[cfg(not(unix))]
 impl<T: Write> OutputWrite for T {}
 
+/// An output data chunk from the mmapped file
+/// Data elements allow output to be performed through write(2)
+/// or through copy_file_range(2).
+#[cfg(unix)]
+#[derive(Clone)]
+struct MmapOutput {
+    in_fast_copy: FastCopy, // Data for fast file copy I/O
+    base_ptr: *const u8,    // Base of the entire mmapped region
+    out_ptr: *const u8,     // Start of the output data chunk
+    len: usize,             // Output data chunk size
+}
+
 /// Abstraction for outputting data, potentially from the mmapped file
-/// Outputs from mmapped data are coallesced and written via a write(2)
-/// system call without any copying if worthwhile.
+/// Outputs from mmapped data are coalesced and written via the Linux
+/// copy_file_range(2) system call without any copying, if possible
+/// and worthwhile.  As a fallback write(2) is used, which requires
+/// the OS to copy data from the mmapped region to the output file
+/// page cache.
 /// All other output is buffered and writen via BufWriter.
 pub struct OutputBuffer {
     out: BufWriter<Box<dyn OutputWrite + 'static>>, // Where to write
     #[cfg(unix)]
-    mmap_ptr: Option<(*const u8, usize)>, // Start and len of chunk to write
+    fast_copy: FastCopy,            // Data for fast file copy ops
+    #[cfg(unix)]
+    max_pending_write: usize,       // Max bytes to keep before flushing
+    #[cfg(unix)]
+    mmap_chunk: Option<MmapOutput>, // Chunk to write
     #[cfg(test)]
-    writes_issued: usize,           // Number of issued write(2) calls
-}
-
-/// Wrapper that issues the write(2) system call
-#[cfg(unix)]
-fn write_syscall(fd: i32, ptr: *const u8, len: usize) -> io::Result<()> {
-    let ret = unsafe { write(fd, ptr as *const c_void, len) };
-    if ret < 0 {
-        Err(std::io::Error::last_os_error())
-    } else {
-        Ok(())
-    }
+    low_level_flushes: usize,       // Number of system call flushes
 }
 
 /// Threshold to use buffered writes for output
@@ -482,21 +561,45 @@ fn write_syscall(fd: i32, ptr: *const u8, len: usize) -> io::Result<()> {
 #[cfg(unix)]
 const MIN_DIRECT_WRITE: usize = 4 * 1024;
 
-/// The maximum size of a pending write buffer
-// Once more than 64k accumulate, issue a write to allow the OS
-// and downstream pipes to handle the output processing in parallel
-// with our processing.
+/// Maximum size of a pending write buffer for files
+// Once more than the specified bytes accumulate, issue a write or
+// a copy_file_range(2).  This is kept high to reduce the number of
+// system calls and (where supported) file extents.
 #[cfg(unix)]
-const MAX_PENDING_WRITE: usize = 64 * 1024;
+const MAX_PENDING_WRITE_FILE: usize = 1024 * 1024;
+
+/// Maximum size of a pending write buffer for non-files (likely pipes)
+// Once more than the specified bytes accumulate, issue a write.
+// This is set to the common size of Linux pipe buffer to maximize
+// throughput and liveness across the pipeline.
+#[cfg(unix)]
+const MAX_PENDING_WRITE_NON_FILE: usize = 64 * 1024;
 
 impl OutputBuffer {
+    #[cfg(not(unix))]
     pub fn new(w: Box<dyn OutputWrite + 'static>) -> Self {
         Self {
             out: BufWriter::new(w),
-            #[cfg(unix)]
-            mmap_ptr: None,
             #[cfg(test)]
-            writes_issued: 0,
+            low_level_flushes: 0,
+        }
+    }
+
+    #[cfg(unix)]
+    pub fn new(w: Box<dyn OutputWrite + 'static>) -> Self {
+        let fast_copy = FastCopy::new(&*w);
+        let max_pending_write = if fast_copy.is_regular {
+            MAX_PENDING_WRITE_FILE
+        } else {
+            MAX_PENDING_WRITE_NON_FILE
+        };
+        Self {
+            out: BufWriter::new(w),
+            fast_copy,
+            max_pending_write,
+            mmap_chunk: None,
+            #[cfg(test)]
+            low_level_flushes: 0,
         }
     }
 
@@ -510,8 +613,11 @@ impl OutputBuffer {
 
     /// Copy the specified file to the output.
     pub fn copy_file(&mut self, path: &PathBuf) -> io::Result<()> {
+        // Flush mmap writes, if any.
         #[cfg(unix)]
-        self.flush_mmap()?; // Flush mmap writes, if any.
+        {
+            self.flush_mmap(WriteRange::Complete)?;
+        }
 
         let file = match File::open(path) {
             Ok(f) => f,
@@ -539,26 +645,63 @@ impl Write for OutputBuffer {
     }
 }
 
+#[cfg(unix)]
+#[derive(Debug, PartialEq)]
+enum WriteRange {
+    Complete, // Write all specified data.
+    Blocks,   // Finish write on a block boundary (to help alignment).
+    None,     // No writing is needed.
+}
+
 #[cfg(unix)]
 impl OutputBuffer {
     /// Schedule the specified output chunk for eventual output
-    pub fn write_chunk(&mut self, chunk: &IOChunk) -> io::Result<()> {
-        match &chunk.content {
-            IOChunkContent::MmapInput { full_span, .. } => {
-                let ptr = full_span.as_ptr();
-                let len = full_span.len();
-
-                if let Some((p, l)) = self.mmap_ptr {
-                    // Coalesce if adjacent
-                    if unsafe { p.add(l) } == ptr && l < MAX_PENDING_WRITE {
-                        self.mmap_ptr = Some((p, l + len));
-                        return Ok(());
+    pub fn write_chunk(&mut self, new_chunk: &IOChunk) -> io::Result<()> {
+        match &new_chunk.content {
+            IOChunkContent::MmapInput {
+                full_span,
+                fast_copy,
+                base,
+                ..
+            } => {
+                let new_ptr = full_span.as_ptr();
+                let new_len = full_span.len();
+
+                // Set whether a flush is needed and whether the
+                // mmap_chunk needs to be reset to the new input.
+                // This avoids calling mmap_chunk (which borrows self)
+                // when old_chunk is already borrowed.
+                let (flush_action, reset) = if let Some(old_chunk) = self.mmap_chunk.as_mut() {
+                    // Coalesce if adjacent.
+                    if unsafe { old_chunk.out_ptr.add(old_chunk.len) } == new_ptr {
+                        // Coalesce.
+                        old_chunk.len += new_len;
+                        if old_chunk.len > self.max_pending_write {
+                            // Too much data; flush some full blocks.
+                            (WriteRange::Blocks, false)
+                        } else {
+                            (WriteRange::None, false)
+                        }
                     } else {
-                        self.flush_mmap()?; // not contiguous
+                        // Not contiguous
+                        (WriteRange::Complete, true)
                     }
+                } else {
+                    // No chunk yet; start a new one.
+                    (WriteRange::None, true)
+                };
+
+                if flush_action != WriteRange::None {
+                    self.flush_mmap(flush_action)?;
+                }
+                if reset {
+                    self.mmap_chunk = Some(MmapOutput {
+                        in_fast_copy: fast_copy.clone(),
+                        base_ptr: *base,
+                        out_ptr: new_ptr,
+                        len: new_len,
+                    });
                 }
-                self.mmap_ptr = Some((ptr, len));
-                Ok(())
             }
 
             IOChunkContent::Owned {
@@ -566,41 +709,62 @@ impl OutputBuffer {
                 has_newline,
                 ..
             } => {
-                self.flush_mmap()?;
+                self.flush_mmap(WriteRange::Complete)?;
                 self.out.write_all(content.as_bytes())?;
                 if *has_newline {
                     self.out.write_all(b"\n")?;
                 }
-                Ok(())
             }
         }
+        Ok(())
     }
 
-    // Flush any pending mmap data
+    /// Flush any pending mmap data.
+    /// Cover specifies whether the written data shall terminate
+    /// on a block boundary.
+    /// This is only respected when copy_file_range is employed.
+    /// Return the number of bytes written.
     #[cfg(unix)]
-    fn flush_mmap(&mut self) -> io::Result<()> {
-        if let Some((ptr, len)) = self.mmap_ptr.take() {
-            if len < MIN_DIRECT_WRITE {
+    fn flush_mmap(&mut self, cover: WriteRange) -> io::Result<()> {
+        if let Some(chunk) = self.mmap_chunk.as_mut() {
+            let written = if chunk.len < MIN_DIRECT_WRITE {
                 // SAFELY treat as &[u8] and write to buffered writer
-                let slice = unsafe { std::slice::from_raw_parts(ptr, len) };
-                return self.out.write_all(slice);
+                let slice = unsafe { std::slice::from_raw_parts(chunk.out_ptr, chunk.len) };
+                self.out.write_all(slice)?;
+                slice.len()
             } else {
                 // Large enough: write directly using zero-copy
-                let fd = self.out.get_ref().as_raw_fd();
                 self.out.flush()?; // sync any buffered data
                 #[cfg(test)]
                 {
-                    self.writes_issued += 1;
+                    self.low_level_flushes += 1;
                 }
-                return write_syscall(fd, ptr, len);
-            }
+                if chunk.in_fast_copy.is_regular && self.fast_copy.is_regular {
+                    portable_copy_file_range(
+                        chunk.out_ptr,
+                        chunk.in_fast_copy.fd,
+                        // Input file offset
+                        unsafe { chunk.out_ptr.offset_from(chunk.base_ptr) } as i64,
+                        self.fast_copy.fd,
+                        chunk.len,
+                        // Alignment block size: the largest of the two
+                        chunk.in_fast_copy.block_size.max(self.fast_copy.block_size),
+                        cover,
+                    )?
+                } else {
+                    reliable_write(self.fast_copy.fd, chunk.out_ptr, chunk.len)?
+                }
+            };
+
+            chunk.len -= written;
+            unsafe { chunk.out_ptr = chunk.out_ptr.add(written) };
         }
         Ok(())
     }
 
     /// Flush everything: pending mmap and buffered data.
     pub fn flush(&mut self) -> io::Result<()> {
-        self.flush_mmap()?; // flush mmap if any
+        self.flush_mmap(WriteRange::Complete)?; // flush mmap if any
         self.out.flush() // then flush buffered data
     }
 }
@@ -630,6 +794,165 @@ impl OutputBuffer {
     }
 }
 
+/// Wrapper that issues the write(2) system calls via write_all
+// This takes care of partial writes and EAGAIN, EWOULDBLOCK, EINTR.
+// Return the number of bytes written.
+#[cfg(unix)]
+fn reliable_write(fd: i32, ptr: *const u8, len: usize) -> std::io::Result<usize> {
+    // A thin Write-compatible wrapper around a raw file descriptor
+    // This allows us to issue and utilize the write_all implementatin.
+    struct FdWriter(RawFd);
+
+    impl Write for FdWriter {
+        fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+            let ret =
+                unsafe { libc::write(self.0, buf.as_ptr() as *const libc::c_void, buf.len()) };
+            if ret < 0 {
+                Err(io::Error::last_os_error())
+            } else {
+                Ok(ret as usize)
+            }
+        }
+
+        fn flush(&mut self) -> io::Result<()> {
+            Ok(())
+        }
+    }
+
+    let mut writer = FdWriter(fd);
+    let buf: &[u8] = unsafe { std::slice::from_raw_parts(ptr, len) };
+    writer.write_all(buf)?;
+    Ok(len)
+}
+
+/// Copy efficiently len data from the input to the output file.
+/// Fall back to write(2) if the platform doesn't support copy_file_range(2).
+/// Return the number of bytes written.
+#[cfg(unix)]
+#[allow(unused_variables)]
+fn portable_copy_file_range(
+    in_ptr: *const u8,
+    in_fd: i32,
+    in_off: libc::off_t,
+    out_fd: i32,
+    len: usize,
+    block_size: usize,
+    cover: WriteRange,
+) -> std::io::Result<usize> {
+    if len == 0 {
+        return Ok(0);
+    }
+
+    #[cfg(all(target_os = "linux", target_env = "gnu"))]
+    {
+        aligned_copy_file_range(in_ptr, in_fd, in_off, out_fd, len, block_size, cover)
+    }
+    #[cfg(not(all(target_os = "linux", target_env = "gnu")))]
+    {
+        reliable_write(out_fd, in_ptr, len)
+    }
+}
+
+/// Copy efficiently len data from the input to the output file.
+/// Handle partial copies and fall back to write(2) if the
+/// file system or options don't support copy_file_range(2).
+/// Return the number of bytes written.
+#[cfg(all(target_os = "linux", target_env = "gnu"))]
+fn reliable_copy_file_range(
+    in_ptr: *const u8,
+    in_fd: i32,
+    mut in_off: libc::off_t,
+    out_fd: i32,
+    len: usize,
+) -> std::io::Result<usize> {
+    let mut pending = len;
+    while pending > 0 {
+        let in_off_ptr: *mut i64 = &mut in_off;
+        let ret = unsafe {
+            libc::copy_file_range(
+                in_fd,
+                in_off_ptr,
+                out_fd,
+                std::ptr::null_mut(), // Use and update output offset
+                pending,
+                0,
+            )
+        };
+        if ret < 0 {
+            let err = io::Error::last_os_error();
+            return match err.raw_os_error() {
+                Some(libc::ENOSYS) | Some(libc::EOPNOTSUPP) | Some(libc::EXDEV) => {
+                    // Fallback to write(2).
+                    reliable_write(out_fd, in_ptr, pending)
+                }
+                _ => Err(err),
+            };
+        } else if ret == 0 {
+            // EOF reached
+            break;
+        } else {
+            pending -= ret as usize;
+        }
+    }
+    Ok(len)
+}
+
+/// Copy efficiently len data from the input to the output file.
+/// Try to call copy_file_range(2) on block-aligned data, so
+/// as to help the filesystem maintain cross-file extents.
+/// Return the number of bytes written.
+#[cfg(all(target_os = "linux", target_env = "gnu"))]
+fn aligned_copy_file_range(
+    mut in_ptr: *const u8,
+    in_fd: i32,
+    mut in_off: libc::off_t,
+    out_fd: i32,
+    len: usize,
+    block_size: usize,
+    cover: WriteRange,
+) -> std::io::Result<usize> {
+    // 1. Get current output offset.
+    let res = unsafe { libc::lseek(out_fd, 0, libc::SEEK_CUR) as i64 };
+    if res < 0 {
+        return Err(std::io::Error::last_os_error());
+    }
+    let out_off = res as usize;
+    let mut pending = len;
+
+    // Obtain head alignment.
+    // Bytes to end of block:
+    let remainder = in_off as usize % block_size;
+    // Bytes to write (block_size becomes 0):
+    let head_align = (block_size - remainder) % block_size;
+
+    if !(out_off + head_align).is_multiple_of(block_size) {
+        // No hope of alignment, so just copy everything.
+        return reliable_copy_file_range(in_ptr, in_fd, in_off, out_fd, pending);
+    }
+
+    if head_align > 0 {
+        // Align the two files on a block boundary.
+        let head_len = head_align.min(pending);
+        reliable_write(out_fd, in_ptr, head_len)?;
+        in_ptr = unsafe { in_ptr.add(head_len) };
+        in_off += head_len as i64;
+        pending -= head_len;
+    }
+
+    // Copy aligned blocks.
+    let aligned_len = pending - (pending % block_size);
+    let _ = reliable_copy_file_range(in_ptr, in_fd, in_off, out_fd, aligned_len);
+    pending -= aligned_len;
+
+    // Copy tail if needed.
+    if pending > 0 && cover == WriteRange::Complete {
+        in_ptr = unsafe { in_ptr.add(aligned_len) };
+        pending -= reliable_write(out_fd, in_ptr, pending)?;
+    }
+
+    Ok(len - pending)
+}
+
 // Usage example (never compiled)
 #[cfg(any())]
 pub fn main() -> io::Result<()> {
@@ -654,9 +977,11 @@ mod tests {
     use std::fs;
     #[cfg(unix)]
     use std::fs::File;
-    #[cfg(unix)]
+    #[cfg(all(target_os = "linux", target_env = "gnu"))]
     use std::io::{self, Write};
+    use std::io::{Seek, SeekFrom};
     use tempfile::NamedTempFile;
+    use tempfile::tempfile;
 
     /// Helper: produce a 4k-byte Vec of `'.'`s ending in `'\n'`.
     #[cfg(unix)]
@@ -667,6 +992,19 @@ mod tests {
         buf
     }
 
+    #[cfg(unix)]
+    pub fn new_content_mmap_input<'a>(
+        content: &'a [u8],
+        full_span: &'a [u8],
+    ) -> IOChunkContent<'a> {
+        IOChunkContent::MmapInput {
+            fast_copy: FastCopy::default(),
+            base: std::ptr::null(),
+            content,
+            full_span,
+        }
+    }
+
     #[test]
     fn test_owned_line_output() -> io::Result<()> {
         let tmp = NamedTempFile::new()?;
@@ -676,7 +1014,7 @@ mod tests {
             out.write_str("foo\n")?;
             out.write_str("bar\n")?;
             out.flush()?;
-            assert_eq!(out.writes_issued, 0);
+            assert_eq!(out.low_level_flushes, 0);
         } // File closes here as it leaves the scope
 
         let contents = fs::read(tmp.path())?;
@@ -715,7 +1053,7 @@ mod tests {
         }
         out.flush()?;
 
-        assert_eq!(out.writes_issued, 0);
+        assert_eq!(out.low_level_flushes, 0);
 
         let written = fs::read(&output_path)?;
         assert_eq!(written.as_slice(), mmap_data);
@@ -761,7 +1099,7 @@ mod tests {
         out.flush()?;
 
         // Since all writes are small (<4K), we expect zero zero copy syscalls
-        assert_eq!(out.writes_issued, 0);
+        assert_eq!(out.low_level_flushes, 0);
 
         // Read both files back and compare
         let expected = {
@@ -806,7 +1144,7 @@ mod tests {
         assert_eq!(nline, 3);
 
         out.flush()?;
-        assert_eq!(out.writes_issued, 1);
+        assert_eq!(out.low_level_flushes, 1);
 
         // Verify that files match:
         let expected = fs::read(&input_path)?;
@@ -845,7 +1183,7 @@ mod tests {
         assert_eq!(nline, 4);
 
         out.flush()?;
-        assert_eq!(out.writes_issued, 1);
+        assert_eq!(out.low_level_flushes, 1);
 
         // Verify that files match:
         let expected = fs::read(&input_path)?;
@@ -880,7 +1218,7 @@ mod tests {
         assert_eq!(nline, 3);
 
         out.flush()?;
-        assert_eq!(out.writes_issued, 0);
+        assert_eq!(out.low_level_flushes, 0);
 
         // Verify that files match:
         let expected = fs::read(&input_path)?;
@@ -915,7 +1253,7 @@ mod tests {
         assert_eq!(nline, 3);
 
         out.flush()?;
-        assert_eq!(out.writes_issued, 0);
+        assert_eq!(out.low_level_flushes, 0);
 
         // Verify that files match:
         let expected = fs::read(&input_path)?;
@@ -931,8 +1269,9 @@ mod tests {
         let mut input = NamedTempFile::new()?;
         write!(input, "first line\nsecond line\n")?;
         let dot_line = make_dot_line_4k();
-        // Write 64k + 16k to ensure one flush when writing
-        for _i in 0..20 {
+        // Create more than MAX... to ensure one flush when writing
+        let nline_in_file = MAX_PENDING_WRITE_FILE / dot_line.len() + 4;
+        for _i in 0..nline_in_file {
             input.write_all(&dot_line)?;
         }
         input.flush()?;
@@ -946,21 +1285,25 @@ mod tests {
         let output_path = output.path().to_path_buf();
         let out_file = File::create(&output_path)?;
 
-        // Wrap it in your OutputBuffer and run the loop:
+        // Wrap it in OutputBuffer and run the loop:
         let mut out = OutputBuffer::new(Box::new(out_file));
-        let mut nline = 0;
+        let mut nline_written = 0;
         while let Some((chunk, _last_line)) = reader.get_line()? {
             out.write_chunk(&chunk)?;
-            nline += 1;
+            nline_written += 1;
         }
-        assert_eq!(nline, 22);
+        assert_eq!(nline_written, nline_in_file + 2);
 
         out.flush()?;
-        assert_eq!(out.writes_issued, 2);
+        assert_eq!(out.low_level_flushes, 2);
 
         // Verify that files match:
         let expected = fs::read(&input_path)?;
         let actual = fs::read(&output_path)?;
+
+        // Fast fail with a readable message, without displaying large output
+        assert_eq!(actual.len(), expected.len());
+
         assert_eq!(actual, expected);
         Ok(())
     }
@@ -1126,7 +1469,7 @@ mod tests {
     fn test_mmap_newline_terminated() {
         let content = b"line";
         let full_span = b"line\n";
-        let chunk = IOChunk::from_content(IOChunkContent::MmapInput { content, full_span });
+        let chunk = IOChunk::from_content(new_content_mmap_input(content, full_span));
         assert!(chunk.is_newline_terminated());
     }
 
@@ -1135,7 +1478,7 @@ mod tests {
     fn test_mmap_not_newline_terminated() {
         let content = b"line";
         let full_span = b"line";
-        let chunk = IOChunk::from_content(IOChunkContent::MmapInput { content, full_span });
+        let chunk = IOChunk::from_content(new_content_mmap_input(content, full_span));
         assert!(!chunk.is_newline_terminated());
     }
 
@@ -1144,7 +1487,7 @@ mod tests {
     fn test_mmap_empty() {
         let content = b"";
         let full_span = b"";
-        let chunk = IOChunk::from_content(IOChunkContent::MmapInput { content, full_span });
+        let chunk = IOChunk::from_content(new_content_mmap_input(content, full_span));
         assert!(!chunk.is_newline_terminated());
     }
 
@@ -1178,7 +1521,7 @@ mod tests {
         let content = b"mmap string";
         let full_span = b"mmap string\n";
 
-        let mut chunk = IOChunk::from_content(IOChunkContent::MmapInput { content, full_span });
+        let mut chunk = IOChunk::from_content(new_content_mmap_input(content, full_span));
 
         let result = chunk.ensure_owned();
         assert!(result.is_ok());
@@ -1202,7 +1545,7 @@ mod tests {
         let content = b"no newline";
         let full_span = b"no newline";
 
-        let mut chunk = IOChunk::from_content(IOChunkContent::MmapInput { content, full_span });
+        let mut chunk = IOChunk::from_content(new_content_mmap_input(content, full_span));
 
         let result = chunk.ensure_owned();
         assert!(result.is_ok());
@@ -1226,7 +1569,7 @@ mod tests {
         let content = b"bad\xFFutf8";
         let full_span = b"bad\xFFutf8\n";
 
-        let mut chunk = IOChunk::from_content(IOChunkContent::MmapInput { content, full_span });
+        let mut chunk = IOChunk::from_content(new_content_mmap_input(content, full_span));
 
         let result = chunk.ensure_owned();
         assert!(result.is_err());
@@ -1255,7 +1598,7 @@ mod tests {
     fn test_fields_mut_on_mmap_input_valid_utf8() {
         let content = b"foo";
         let full_span = b"foo\n";
-        let mut chunk = IOChunk::from_content(IOChunkContent::MmapInput { content, full_span });
+        let mut chunk = IOChunk::from_content(new_content_mmap_input(content, full_span));
 
         {
             let (s, _) = chunk.fields_mut().unwrap();
@@ -1270,7 +1613,7 @@ mod tests {
     fn test_fields_mut_on_utf8_multibyte() {
         let content = "Ζωντανά!".as_bytes();
         let full_span = "Ζωντανά!\n".as_bytes();
-        let mut chunk = IOChunk::from_content(IOChunkContent::MmapInput { content, full_span });
+        let mut chunk = IOChunk::from_content(new_content_mmap_input(content, full_span));
 
         let (s, _) = chunk.fields_mut().unwrap();
         s.push_str(" Δεδομένα");
@@ -1283,10 +1626,298 @@ mod tests {
     fn test_fields_mut_invalid_utf8() {
         let content = b"abc\xFF"; // invalid UTF-8
         let full_span = b"abc\xFF\n";
-        let mut chunk = IOChunk::from_content(IOChunkContent::MmapInput { content, full_span });
+        let mut chunk = IOChunk::from_content(new_content_mmap_input(content, full_span));
 
         let result = chunk.fields_mut();
         assert!(result.is_err());
         assert!(format!("{}", result.unwrap_err()).contains("invalid utf-8"));
     }
+
+    #[cfg(unix)]
+    #[test]
+    fn fastcopy_regular_file() {
+        let mut file = tempfile().expect("create temp file");
+        writeln!(file, "hello").unwrap();
+
+        let fc = FastCopy::new(&file);
+
+        assert!(fc.is_regular, "expected regular file");
+        assert!(fc.block_size > 0, "block size should be > 0");
+    }
+
+    #[cfg(unix)]
+    #[test]
+    fn fastcopy_non_regular_devnull() {
+        let file = File::open("/dev/null").expect("open /dev/null");
+
+        let fc = FastCopy::new(&file);
+
+        assert!(
+            !fc.is_regular,
+            "expected /dev/null to be reported as non-regular"
+        );
+    }
+
+    #[cfg(unix)]
+    #[test]
+    fn test_reliable_write_to_file() {
+        // Create an anonymous temporary file.
+        let mut file = tempfile().expect("failed to create tempfile");
+
+        // The data to write
+        let data = b"hello reliable_write!";
+        let fd = file.as_raw_fd();
+
+        // Call our function
+        let written = reliable_write(fd, data.as_ptr(), data.len()).expect("reliable_write failed");
+
+        assert_eq!(written, data.len());
+
+        // Rewind file and read back contents
+        file.rewind().unwrap();
+        let mut buf = Vec::new();
+        file.read_to_end(&mut buf).unwrap();
+
+        assert_eq!(&buf, data);
+    }
+
+    #[cfg(unix)]
+    #[test]
+    fn test_reliable_write_handles_partial_write() {
+        use std::thread;
+        use std::time::Duration;
+
+        // Create a pipe
+        let mut fds = [0; 2];
+        assert_eq!(unsafe { libc::pipe(fds.as_mut_ptr()) }, 0);
+        let read_fd = fds[0];
+        let write_fd = fds[1];
+
+        // Spawn a reader that drains slowly
+        thread::spawn(move || {
+            let mut buf = [0u8; 1024];
+            loop {
+                let n = unsafe { libc::read(read_fd, buf.as_mut_ptr() as *mut _, buf.len()) };
+                if n <= 0 {
+                    break;
+                }
+                thread::sleep(Duration::from_millis(10));
+            }
+        });
+
+        // Prepare data bigger than pipe capacity to force partial writes
+        let big = vec![b'x'; 200_000];
+
+        // This will require multiple kernel writes, exercising write_all
+        let n = reliable_write(write_fd, big.as_ptr(), big.len()).expect("reliable_write failed");
+
+        assert_eq!(n, big.len());
+
+        unsafe { libc::close(write_fd) };
+    }
+
+    #[test]
+    #[cfg(all(target_os = "linux", target_env = "gnu"))]
+    fn test_aligned_copy_cover_includes_tail() {
+        let mut infile = tempfile().unwrap();
+        let mut outfile = tempfile().unwrap();
+
+        // Input data = 10 bytes
+        let data = b"abcdefghij";
+        infile.write_all(data).unwrap();
+        infile.rewind().unwrap();
+
+        let in_fd = infile.as_raw_fd();
+        let out_fd = outfile.as_raw_fd();
+        let in_ptr = data.as_ptr();
+
+        // Copy with block size 4, cover = Complete
+        let copied = aligned_copy_file_range(
+            in_ptr,
+            in_fd,
+            0,
+            out_fd,
+            data.len(),
+            4,
+            WriteRange::Complete,
+        )
+        .unwrap();
+
+        assert_eq!(copied, data.len());
+
+        // Verify outfile contents = full input data
+        outfile.rewind().unwrap();
+        let mut buf = Vec::new();
+        outfile.read_to_end(&mut buf).unwrap();
+        assert_eq!(&buf, data);
+    }
+
+    #[test]
+    #[cfg(all(target_os = "linux", target_env = "gnu"))]
+    fn test_aligned_copy_blocks_skips_tail() {
+        let mut infile = tempfile().unwrap();
+        let mut outfile = tempfile().unwrap();
+
+        // Input data = 10 bytes
+        let data = b"abcdefghij";
+        infile.write_all(data).unwrap();
+        infile.rewind().unwrap();
+
+        let in_fd = infile.as_raw_fd();
+        let out_fd = outfile.as_raw_fd();
+        let in_ptr = data.as_ptr();
+
+        // Copy with block size 4, cover = Blocks
+        let copied =
+            aligned_copy_file_range(in_ptr, in_fd, 0, out_fd, data.len(), 4, WriteRange::Blocks)
+                .unwrap();
+
+        // Only full 8 bytes (2 blocks of 4) should be copied
+        assert_eq!(copied, 8);
+
+        // Verify outfile contents = only the first 8 bytes
+        outfile.rewind().unwrap();
+        let mut buf = Vec::new();
+        outfile.read_to_end(&mut buf).unwrap();
+        assert_eq!(&buf, b"abcdefgh");
+    }
+
+    ///////////////////////////////
+    // Unit tests for write_chunk()
+    ///////////////////////////////
+
+    fn new_for_test() -> (OutputBuffer, std::fs::File) {
+        let file = tempfile().unwrap();
+        let buf = OutputBuffer {
+            out: BufWriter::new(Box::new(file.try_clone().unwrap())),
+            #[cfg(unix)]
+            fast_copy: FastCopy {
+                fd: -1,
+                is_regular: false,
+                block_size: 2,
+            },
+            #[cfg(unix)]
+            max_pending_write: 8,
+            #[cfg(unix)]
+            mmap_chunk: None,
+            low_level_flushes: 0,
+        };
+        (buf, file)
+    }
+
+    #[cfg(unix)]
+    fn make_mmap_chunk(bytes: &'static [u8]) -> IOChunk<'static> {
+        IOChunk {
+            utf8_verified: Cell::new(true),
+            content: IOChunkContent::MmapInput {
+                fast_copy: FastCopy {
+                    fd: -1,
+                    is_regular: false,
+                    block_size: 1,
+                },
+                base: bytes.as_ptr(),
+                content: bytes,
+                full_span: bytes,
+            },
+        }
+    }
+
+    fn make_owned_chunk(s: &str, has_nl: bool) -> IOChunk<'_> {
+        IOChunk {
+            utf8_verified: Cell::new(true),
+            content: IOChunkContent::Owned {
+                content: s.to_string(),
+                has_newline: has_nl,
+                #[cfg(not(unix))]
+                _phantom: std::marker::PhantomData,
+            },
+        }
+    }
+
+    #[cfg(unix)]
+    #[test]
+    fn mmap_new_chunk_single() {
+        let (mut outbuf, _file) = new_for_test(); // OutputBuffer
+
+        let c1 = make_mmap_chunk(b"abc");
+
+        outbuf.write_chunk(&c1).unwrap();
+
+        assert_eq!(outbuf.mmap_chunk.as_ref().unwrap().len, 3);
+    }
+
+    #[cfg(unix)]
+    #[test]
+    fn mmap_new_chunk_and_coalesce() {
+        let (mut outbuf, _file) = new_for_test(); // OutputBuffer
+
+        let backing = b"abcdefgh"; // contiguous buffer
+        let c1 = make_mmap_chunk(&backing[0..4]); // "abcd"
+        let c2 = make_mmap_chunk(&backing[4..8]); // "efgh"
+
+        outbuf.write_chunk(&c1).unwrap();
+        outbuf.write_chunk(&c2).unwrap();
+
+        assert_eq!(outbuf.mmap_chunk.as_ref().unwrap().len, 8);
+    }
+
+    #[test]
+    #[cfg(unix)]
+    fn mmap_not_contiguous_triggers_flush() {
+        let (mut buf, _file) = new_for_test();
+        let backing = b"abcdefghi";
+        let c1 = make_mmap_chunk(&backing[0..4]); // "abcd"
+        // Guaranteed non-coalescable.  Surprisingly, on macOS
+        // passing two strings resulted in coalescible data.
+        let c2 = make_mmap_chunk(&backing[5..9]); // "fghi"
+
+        buf.write_chunk(&c1).unwrap();
+        assert_eq!(buf.mmap_chunk.as_ref().unwrap().len, 4);
+        buf.write_chunk(&c2).unwrap();
+        // No coalescing
+        assert_eq!(buf.mmap_chunk.as_ref().unwrap().len, 4);
+    }
+
+    #[test]
+    #[cfg(unix)]
+    fn mmap_coalesce_and_flush_blocks() {
+        let (mut buf, _file) = new_for_test();
+        buf.max_pending_write = 4;
+        let backing = b"abcdefgh"; // contiguous buffer
+        let c1 = make_mmap_chunk(&backing[0..5]); // "abcde"
+        let c2 = make_mmap_chunk(&backing[5..8]); // "fgh"
+
+        buf.write_chunk(&c1).unwrap();
+        buf.write_chunk(&c2).unwrap();
+        // After a flush
+        assert_eq!(buf.mmap_chunk.as_ref().unwrap().len, 0);
+    }
+
+    #[test]
+    fn owned_without_newline() {
+        let (mut buf, mut file) = new_for_test();
+        let chunk = make_owned_chunk("hello", false);
+        buf.write_chunk(&chunk).unwrap();
+
+        buf.out.flush().unwrap();
+        file.seek(SeekFrom::Start(0)).unwrap();
+        let mut out = String::new();
+        file.read_to_string(&mut out).unwrap();
+
+        assert_eq!(out, "hello");
+    }
+
+    #[test]
+    fn owned_with_newline() {
+        let (mut buf, mut file) = new_for_test();
+        let chunk = make_owned_chunk("world", true);
+        buf.write_chunk(&chunk).unwrap();
+
+        buf.out.flush().unwrap();
+        file.seek(SeekFrom::Start(0)).unwrap();
+        let mut out = String::new();
+        file.read_to_string(&mut out).unwrap();
+
+        assert_eq!(out, "world\n");
+    }
 }
diff --git a/tests/by-util/test_sed.rs b/tests/by-util/test_sed.rs
index 433cd70..b8ec7d0 100644
--- a/tests/by-util/test_sed.rs
+++ b/tests/by-util/test_sed.rs
@@ -93,6 +93,21 @@ fn test_no_script_file() {
     }
 }
 
+/// Test correct functioning of copy_file_range.
+#[test]
+fn test_multiple_input_files() {
+    new_ucmd!()
+        .args(&[
+            "-e",
+            "",
+            "input/dots-64k.txt",
+            "input/no-new-line.txt",
+            "input/dots-64k.txt",
+        ])
+        .succeeds()
+        .stdout_is_fixture("output/multiple_input_files");
+}
+
 #[test]
 fn test_delete_stdin() {
     for fixture in INPUT_FILES {
@@ -114,6 +129,20 @@ fn test_delete_file() {
     }
 }
 
+#[test]
+#[cfg(unix)]
+fn test_special_file() {
+    for fixture in INPUT_FILES {
+        // To test path avoiding copy_file_range
+        let devnull = fs::File::create("/dev/null").unwrap();
+
+        new_ucmd!()
+            .args(&["-e", "", fixture])
+            .set_stdout(devnull)
+            .succeeds();
+    }
+}
+
 /// Create a new test function to verify an execution for specified output.
 macro_rules! check_output {
     ($name:ident, $args:expr) => {
diff --git a/tests/fixtures/sed/output/multiple_input_files b/tests/fixtures/sed/output/multiple_input_files
new file mode 100644
index 0000000..2529dcc
--- /dev/null
+++ b/tests/fixtures/sed/output/multiple_input_files
@@ -0,0 +1,32 @@
+................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................
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diff --git a/util/benchmark.sh b/util/benchmark.sh
index a3b6232..17d405c 100755
--- a/util/benchmark.sh
+++ b/util/benchmark.sh
@@ -22,7 +22,7 @@ SCRIPTS=tests/fixtures/sed/script
 # Run hyperfine with the specified name and command and collect the results.
 bench_run()
 {
-  if hyperfine --command-name "$1" --warmup 2 --export-csv out.csv "$2" ; then
+  if hyperfine --command-name "$1" --warmup 2 --export-csv out.csv --output ./out.txt "$2" ; then
     # Output the results sans-heading.
     sed 1d out.csv >>"$OUT"
   else
@@ -30,7 +30,7 @@ bench_run()
     echo "$1,,,,,,," >>"$OUT"
   fi
 
-  rm out.csv
+  rm out.csv out.txt
 }
 
 # Shared heading