How does cat 'know' the optimum block size to use?

Question

From reading this, it seems that when copying data to a different hard drive, cat automatically uses the optimum block size (or very near it).

I wonder how it determines the optimum block size, and whether the method cat uses can be applied to dd somehow.

Answer 1

The main loop of GNU cat, in the simplest case is (function simple_cat from cat.c):

while (true)
    {
        /* Read a block of input. */
        n_read = safe_read (input_desc, buf, bufsize);

        /* ... */
    }

Then the question becomes "how is bufsize set?" The answer is it's using io_blksize (insize = io_blksize (stat_buf)), which is defined as follows:

io_blksize (struct stat sb)
{
  return MAX (IO_BUFSIZE, ST_BLKSIZE (sb));
}

where ST_BLKSIZE gives the operating system's idea of the file system's preferred I/O block size (as accessed using stat), and IO_BUFSIZE is defined as 128*1024 (128KB). Here is an excerpt of the Linux stat syscall documentation:

blksize_t st_blksize; /* blocksize for file system I/O */ (...)

The st_blksize field gives the "preferred" blocksize for efficient
file system I/O.   (Writing to a file in smaller  chunks may cause
an inefficient read-modify-rewrite.)

So it seems that GNU cat will read in blocks of 128KB or the file system's recommended I/O block size, whichever is larger.