Phillip Susi posted on Wed, 03 Dec 2014 22:09:29 -0500 as excerpted: > Are you sure the kernel only gains awareness of btrfs volumes when user > space runs btrfs device scan? If that is so then that means you can not > boot from a multi device btrfs root without using an initramfs. I > thought the kernel auto scanned all devices if you tried to mount a > multi device volume, but if this is so, then yes, the udev rules could > be fixed to not call btrfs device scan on an lvm snapshot.
That has indeed been the case in the past, and to my knowledge remains the case. Unless it has changed in the last cycle or two (and I've not seen patches to that effect on the list nor any hint of such, so I doubt it) the kernel doesn't do any such scanning without userspace telling it to. The device= mount option can be used instead, but it didn't work with rootflags= on the kernel commandline last I tried so for a multidevice btrfs root, yes, an initramfs/initrd is required. Which is why I'm running an initramfs for the first time since I've switched to btrfs raid1 mode root, as I quit with initrds back before initramfs was an option. An initramfs appended to the kernel image beats a separate initrd, but I'd still love to see the kernel commandline parsing fixed so it broke at the correct = in rootflags=device= (which seemed to be the problem, the kernel then didn't seem to recognize rootflags at all, as it was apparently seeing it as a parameter called rootflags=device, instead of rootflags), so I could be rid of the initramfs again. FWIW, I'm using dracut to generate the cpio archive, which with the right kernel config options set, the kernel build process then appends to the kernel. Dracut btrfs module enabled of course, most of the rest force- disabled as I run a monolithic kernel so don't need module loading, etc. -- Duncan - List replies preferred. No HTML msgs. "Every nonfree program has a lord, a master -- and if you use the program, he is your master." Richard Stallman -- 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
