cp: document sparse_copy_without_hole more

src/uu/cp/src/platform/linux.rs

@@ -141,7 +141,9 @@ where
     }
     let src_fd = src_file.as_raw_fd();
     let mut current_offset: isize = 0;
-    let step = std::cmp::min(size, 16 * 1024 * 1024) as usize; // 16 MiB
+    // Maximize the data read at once to 16 MiB to avoid memory hogging with large files
+    // 16 MiB chunks should saturate an SSD
+    let step = std::cmp::min(size, 16 * 1024 * 1024) as usize;
     let mut buf: Vec<u8> = vec![0x0; step];
     loop {
         let result = unsafe { libc::lseek(src_fd, current_offset.try_into().unwrap(), SEEK_DATA) }
@@ -160,7 +162,9 @@ where
             return Err(std::io::Error::last_os_error());
         }
         let len: isize = hole - current_offset;
+        // Read and write data in chunks of `step` while reusing the same buffer
         for i in (0..len).step_by(step) {
+            // Ensure we don't read past the end of the file or the start of the next hole
             let read_len = std::cmp::min((len - i) as usize, step);
             let buf = &mut buf[..read_len];
             src_file.read_exact_at(buf, (current_offset + i) as u64)?;