On 12/16/17 10:23, Stuart Henderson wrote: > On 2017/12/16 04:00, [email protected] wrote: >> > On 2017-12-15, Stuart wrote: >> > You can use dd to write zeroes over the start of the device to overwrite >> > the partition table and disklabel, I'm not surehow far you have to go >> > but would expect "dd if=/dev/zero of=/dev/rsdXc bs=1m count=8" to do >> > the trick (with the correct disk number here ^ obviously). >> .. >> >> Hi, >> >> In the past I thought the principle was that you wipe the disklabel by >> overwriting the first megabyte (bs=1m count=1). >> >> What is the safe practice really, 1MB or 8MB? >> >> Tinker > > I'd rather err on the side of wiping too much when I'm clearing out > the old partition table and disklabel, it doesn't take much longer. > When I'm partitioning SSDs I usually align to 2048 sectors anyway > in an attempt to align to flash erase blocks, so just wiping the > first 1MB wouldn't be enough in that case. >
yeah. You are thinking too hard about this...(and possibly, about the wrong things). Personally, I leave off the bs=1m count=x and hit enter, and in the period of time it would take me to type "bs=1m count=x" accurately, I hit CTRL-C. More than enough will have been cleared. Obviously, that's not a scriptable answer. :) More critical is WHAT you zero. The point is to hit the softraid metadata. It's very possible to zero the start of a drive and MISS the softraid metadata if the softraid partition is not at the beginning of the disk. Let's say you want to encrypt the /home partition, but have the system free boot so something/someone can log in and unlock the /home partition. So let's say the /home partition is 10G into the disk. You zero the start of the drive, but not 10G in. Now, you rebuild the system and partition it exactly as before. Hello! There's your softraid metadata coming back from the deleted. So ... rather than zeroing the drive (rsdXc), I'd recommend zeroing the RAID PARTITION(s) after laying them out in disklabel, before trying to use bioctl to assemble them. So ... if sd0m is your softraid partition, zero out the first MB (plus or minus a lot) of /dev/rsd0m. IF you are using the entire disk for your softraid partitions, then Stuart's c partition clear is functionally the same, but if you are using just part of the disk, really...zero that partition. Nick.
