On Sat, Dec 14, 2024 at 05:08:54PM -0500, Demi Marie Obenour wrote: > > From what I can tell, the Discoverable Partition Table specification, > > at least as defined here[1] only supports explicit file system types > > supplied by the GPT partition table. > > > > [1] https://uapi-group.org/specifications/specs/discoverable_partitions_specification/ > > It’s the other way around: the GPT only provides the mountpoint, > never the type. That’s why I filed an issue [1] asking for > per-filesystem-type UUIDs. Bleah, you're right. Other partition tables, including MBR(!) used the "partiton type" to be the kind of file system. (For example, 0x07h meant OS/2, 0x09 meant QNX/Coherent/OS-9, 0x0Bh meant FAT32 with CHS addressing, 0x0Ch meat FAT32 with LBA, 39h meant Plan9, etc.) When I saw "partition type" in the UEFI spec, I thought they were seeing the path of wisdom and moving away from in-band signaling to an explicit type specification --- but you're right, looking at the UEFI spec more closely, it's about how the file system is to be used, not the file system type. (It's really not even the mount point, since 773f91ef-66d4-49b5-bd83-d683bf40ad16 means "per-user home partition", but since the UUID doesn't specify the username, you would't know whether it was supposed to be mounted in /home/lucy, or /home/snoopy, or /home/charlie_brown. Yelch....) > I’m not too worried about this, and instead am of the opinion that it > needs to be fixed on the blkid side (by ignoring the ZFS superblock). I disagree; blkid's *job* is to detect the file system type, and just ignoring all ZFS superblocks means that it won't be able to detect ZFS file systems, which would be sad. And having some kind of arbitrary preference where blkid were to say, "well, if it's ambiguous whether a block device is ext4 or btrfs or ZFS, I'll just arbitrarily say ext4 because I like ext4 more" is well, arbitrary. The best way to solve this is to either have users use "wipefs -a /dev/hdXX" before running a mkfs program, but in the spirit of being kind to users[1] who don't know about wipefs, or for distro installers that don't bother to call wipefs, I'm perfectly happy to teach mkfs.ext4 how to make the right thing happen automatically. I just need to know how to zap ZFS superbloks. BTW, in practice this happens automatically for SSD's, since we will call BLKDISCARD on the entire device, for better FTL GC performance. But for HDD's, we will need to explicitly write zeroes in the correct location. Cheers, - Ted [1] Using a variation from Struck and White's "The Elements of Style" where they said, "always write with a deep empathy towards the reader", we should strive to program with deep empathy towards the user. :-)
