Re: bcwipe won't wipe a block device...

On Fri, 20 Feb 2004, John-Mark Gurney wrote:

> Peter Jeremy wrote this message on Fri, Feb 20, 2004 at 18:23 +1100:
> > Summary: bcwipe is trying to read 1 byte from an offset of 2^N-1.
> > FreeBSD no longer has block devices (since 4.0) - /dev/da0 is a
> > character device.  Character devices can only be read in blocksize
> > units (typically 512 bytes for disks).  You need to fix bcwipe to
> > handle character devices.
>
> this can be done by using the fstat call on the device node.. it will
> provide both the proper block size (so it'll work w/ md swap backed
> devices that have a 4k block size) and device size...

Um, this can not be done by using the fstat call on the device node.
fstat() on devices always gives 0 for the device size and is little
better for the block size.  The block size is always the nominal
value PAGE_SIZE for non-disk devices and is not always the nominal
value BLKDEV_IOSIZE for disk devices.  From vfs_vnops.c:

% 	} else if (vn_isdisk(vp, NULL)) {
% 		sb->st_blksize = vp->v_rdev->si_bsize_best;
% 		if (sb->st_blksize < vp->v_rdev->si_bsize_phys)
% 			sb->st_blksize = vp->v_rdev->si_bsize_phys;
% 		if (sb->st_blksize < BLKDEV_IOSIZE)
% 			sb->st_blksize = BLKDEV_IOSIZE;
% 	} else {
% 		sb->st_blksize = PAGE_SIZE;
% 	}

Here si_bsize_phys is the sector size of drivers or GEOM set it, else
it is defaulted to DEV_BSIZE in spec_open().  si_bsize_bestt is fully
rotted garbage -- it is only used above, but never set except using
bzero() to 0.  Since sector sizes are normally smaller than BLKDEV_IOSIZE
and BLKDEV_IOSIZE is normally PAGE_SIZE, the above normally sets
st_blksize to PAGE_SIZE for all devices.

Bruce