Re: Block size: who decides?

[Paul Gearon <pag@PISoftware.com>]

On Sun, 1 Jul 2001, Peter Chapin wrote:
> I suppose it's up to me to decide on the size of my allocation units.
> However, how do I find out, in my userspace program, how large the sectors
> are (perhaps I don't need to know that there)? Also how can my userspace
> program find out how large the partition is?

Disclaimer:  I don't really know what I'm talking about here.  :-)


Try looking up the source code to mke2fs, since this is the sort of thing
you're trying to do anyway.  There's a file called getsize.c in the
lib/ext2fs directory.

You'll find that BLKGETSIZE is defined in linux/fs.h as:
  _IO(0x12,96)
(otherwise getsize.c will define it for you)

Then the function ext2fs_get_device_size() uses this BLKGETSIZE with
ioctl() to get the size.  Dramatically simplified, it basically says:

errcode_t ext2fs_get_device_size(const char *file, int blocksize,
         blk_t *retblocks)
{
  int fd;
  unsigned long size;
  fd = open(file, O_RDONLY);
  if (ioctl(fd, BLKGETSIZE, &size) >= 0) {
    close(fd);
    *retblocks = size / (blocksize / 512);
    return 0;
  }
  /* else use a binary search to find the size */
}


There's a DEBUG main() function which assumes that blocksize is 1024, but
mke2fs uses a better, though more complicated approach (including the
option of letting the user define it for themselves).

The binary search part is also pretty simple.  It just does a seek and a
read of a single byte to check if an offset is valid.  If it's valid then
it increases the size, otherwise it decreases the size.  Ted makes a
comment that this is REALLY not the way you want to do it, but if ioctl()
doesn't work then you don't have too much of a choice.  :-)

Anyway, everything you need should be in there, including the option to
find the blocksize automatically.

Regards,
Paul Gearon

Software Engineer                Telephone:   +61 7 3876 2188
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