Hi, Short version: # mkfs.any /dev/vda4 unable to open /dev/vda4: Device or resource busy Curiously /dev/vda4 is just a blank partition, not in use by anything that I'm aware of. And gdisk is allowed to modify the GPT on /dev/vda without complaint. This is a snippet from strace of the above command at the failure point: openat(AT_FDCWD, "/dev/vda4", O_RDWR|O_EXCL) = -1 EBUSY (Device or resource busy) write(2, "ERROR: ", 7ERROR: ) = 7 write(2, "unable to open /dev/vda4: Device"..., 49unable to open /dev/vda4: Device or resource busy) = 49 write(2, "\n", 1 ) Long version: 1. Create an image with xorriso. Result is an ISO 9660 image that also contains an MBR and GPT. Together, these are all in conflict but permit magical indirection to successfully boot a computer whether it's UEFI or BIOS, on optical or USB sticks. 2. Write this image to a USB stick using cat or dd. 3. The horrible idea occurs to use the large pile of extra space on the USB stick for persistence. Therefore a partition must be added. 4. Given the MBR is not a single partition PMBR, its existence invalidates the GPT. And at least gdisk doesn't want to modify the GPT at all. But I can create a new GPT and PMBR, and in effect merge the old MBR and GPT entries to protect the two El Torito EFI system partitions, and the ISO 9660 image as a whole. Write this out to /dev/vda without any complaint, and the resulting image still boots BIOS and UEFI systems. 5. Upon reboot, /dev/vda4 exists, a large empty partition. The idea is to format it, but all of the following commands fail with EBUSY: # mkdosfs /dev/vda4 # mkfs.ext4 /dev/vda4 # mkfs.xfs /dev/vda4 # mkfs.btrfs /dev/vda4 # btrfs device add /dev/vda4 /run/rootfsbase [1] 6. Color me confused. Granted, despite cleaning up the conflicting MBR and GPT, this is still an odd duck. ISO 9660 ostensibly is a read-only format, and /proc/mounts shows /dev/vda /run/initramfs/live iso9660 ro,relatime,nojoliet,check=s,map=n,blocksize=2048 0 0 So it sees the whole vda device as iso9660 and ro? But permits gdisk to modify some select sectors on vda? I admit it's an ambiguous image. Is it a duck or is it a rabbit? And therefore best to just look at it, not make modifications to it. Yet /dev/vda is modifiable, where the partitions aren't. Hmm. Thanks, -- Chris Murphy [1] The last one for the future fs-devel trivia pursuit game. There is a mounted btrfs "seed" on /run/rootfsbase, and it's possible to make it a read-write "sprout" merely by adding a writable device. The command implies mkfs and file system resize. Once it completes, it's possible to remount,rw and all changes are directed to this device. -- Chris Murphy
