[SOLVED] Erasing a disk before writing and expanding a fs image

ltkenbo · 07-06-2015, 09:24 AM

I'm writing a small ext4 image I've built to a block device in Linux using the dd utility.

i.e.

Code:

dd if=/root/ext4image.img of=/dev/'blockdevice'

After this, I delete the partition on the device with fdisk and recreate it to use the rest of the disk. I then run fsck to check the disk and then finally resize2fs to resize the file system. This works for me just fine.

My main question with this method is, is it necessary to erase the disk first?

i.e.

Code:

dd if=/dev/zero of=/dev/'blockdevice'

Are there any consequences to not erasing it first?

MensaWater · 07-06-2015, 09:28 AM

Not from an ext4 standpoint. It will recognize the additional space you give it with the resize2fs.

Since you're using a partition you'd have to make sure you see the additional space at OS level before you did the resize2fs of the filesystem. (e.g. After the change use fdisk -l or parted to verify the OS sees the additional space.

Keruskerfuerst · 07-06-2015, 09:30 AM

If you do not erase the disk completly, the data in the area, which was not overwritten by the image, will be kept.

ltkenbo · 07-06-2015, 10:19 AM

Quote:

Originally Posted by Keruskerfuerst

If you do not erase the disk completly, the data in the area, which was not overwritten by the image, will be kept.

So let's say I have a full device with an ext4 file system. I then perform my method of writing this smaller image, then re-partitioning and expanding. It is than possible that the files from the previous full file system are still present in this new one? Where would they be, in lost+found?

Keruskerfuerst · 07-06-2015, 10:30 AM

It is only possible to access the data in the not-overwritten area directly.

ltkenbo · 07-06-2015, 10:33 AM

Oh ok. So I needn't worry about this affecting my "new" file system correct? (This is all I care about)

rknichols · 07-06-2015, 10:36 AM

"Kept" only in the sense of still available for forensic recovery. If you ever do run into a problem where you have to resort to recovery tools like testdisk and photorec, it can be a bit confusing to have them discovering that old data. Other than that and privacy concerns, there's no need to erase the disk.

That said, I generally do zero a hard disk or flash drive (though probably not an SSD) before re-using it.

ltkenbo · 07-06-2015, 10:53 AM

Alright guys thanks this is exactly the info I was needing!