Function io_blksize

src/ioblksize.h:80–111

 */
enum { IO_BUFSIZE = 256 * 1024 };
static inline idx_t
io_blksize (struct stat const *st)
{
  /* Treat impossible blocksizes as if they were IO_BUFSIZE.  */
  idx_t blocksize = STP_BLKSIZE (st) <= 0 ? IO_BUFSIZE : STP_BLKSIZE (st);

  /* Use a blocksize of at least IO_BUFSIZE bytes, keeping it a
     multiple of the original blocksize.  */
  blocksize += (IO_BUFSIZE - 1) - (IO_BUFSIZE - 1) % blocksize;

  /* For regular files we can ignore the blocksize if we think we know better.
     ZFS sometimes understates the blocksize, because it thinks
     apps stupidly allocate a block that large even for small files.
     This misinformation can cause coreutils to use wrong-sized blocks.
     Work around some of the performance bug by substituting the next
     power of two when the reported blocksize is not a power of two.  */
  if (S_ISREG (st->st_mode) && blocksize & (blocksize - 1))
    {
      int leading_zeros = stdc_leading_zeros_ull (blocksize);
      if (IDX_MAX < ULLONG_MAX || leading_zeros)
        {
          unsigned long long power = 1ull << (ULLONG_WIDTH - leading_zeros);
          if (power <= IDX_MAX)
            blocksize = power;
        }
    }

  /* Don’t go above the maximum number of bytes
     to read or write in a single system call,
     as that is asking for trouble.  */
  return MIN (SYS_BUFSIZE_MAX, blocksize);
}