This is almost definitely a bug in GRUB, but I wanted to get the btrfs mailing list opinion first.
Symptoms: I have a btrfs raid1 /boot and root filesystem. Ever since I replaced a drive, when I run the grub utilities to create my grub.cfg and install to boot sector, it only recognizes one of the drives. $ sudo grub-probe /boot/grub -t device /dev/mapper/VG_BTRFS2-LV_BOOT2 $ sudo grub-probe /boot/grub -t bios_hints lvmid/gEfhOx-J9hr-8tkA-OgjD-Aqqu-XR2T-sFB4me/oNnMDp-Rit5-P0qs-QZlf-bQQe-tZU7-Wwmz8z This also prevents boot if the above drive is disconnected. Grub error in locating lvmid/gEfhOx-J9hr-8tkA-OgjD-Aqqu-XR2T-sFB4me/oNnMDp-Rit5-P0qs-QZlf-bQQe-tZU7-Wwmz8z Boot works fine if both drives, or only the above drive is present. Before drive replacement, the above commands returned both drives that were part of the RAID1 mirror. I never tried booting with a device disconnected, but both showed up in my grub.cfg. Replacement was not standard since the prior drive was developing bad sectors, but had not failed. Replacement was done by adding a third disk to the mirror, then removing the 1st disk. Probable Cause: To determine the boot drive, grub-probe and grub-mkimage make several ioctl in osdep/linux/getroot.c: grub_find_root_devices_from_btrfs A call to BTRFS_IOC_FS_INFO gets the max_id and num_devices. It then iterates from 1 to max_id, calling BTRFS_IOC_DEV_INFO to get the path. For my system, max_id = 3 and num_devices = 2. Requesting BTRFS_IOC_DEV_INFO for device 1 yields a "No Such Device". Instead of continuing on to device 2 and 3 (which return without error), grub treats all ioctl errors as fatal, exits the btrfs specific code with a failure, then falls back to generic linux code that only detects the single drive. So, is this a grub bug? If so, any suggestions before I submit to the grub-bug list? Also, as I wait until a fix is published (or I rebuild grub with my own patch), any ideas to workaround this? Info: Kernel: 4.15.0-12-generic #13-Ubuntu SMP (based on 4.15.7 mainline) btrfs-progs: 4.15.1-1 Grub: 2.02~beta2-36ubuntu3.17 $ btrfs fi show Label: none uuid: 84c8e78b-9d7f-4451-966d-3c25154e89b8 Total devices 2 FS bytes used 22.16GiB devid 2 size 100.00GiB used 25.03GiB path /dev/mapper/VG_BTRFS2-LV_ROOT2 devid 3 size 100.00GiB used 25.03GiB path /dev/mapper/VG_BTRFS3-LV_ROOT3 Label: none uuid: 059ab98f-eb63-471d-b099-6561baf39040 Total devices 2 FS bytes used 61.04GiB devid 2 size 200.00GiB used 62.03GiB path /dev/mapper/VG_BTRFS2-LV_HOME2 devid 3 size 200.00GiB used 62.03GiB path /dev/mapper/VG_BTRFS3-LV_HOME3 Label: none uuid: ffe8b1a0-030c-42c2-94f5-b7e8e54b1439 Total devices 2 FS bytes used 342.04MiB devid 2 size 1.00GiB used 693.62MiB path /dev/mapper/VG_BTRFS2-LV_BOOT2 devid 3 size 1.00GiB used 693.62MiB path /dev/mapper/VG_BTRFS3-LV_BOOT3-- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html