On Oct 14, 2013, at 8:33 PM, Andrey Borzenkov <arvidj...@gmail.com> wrote:
> В Mon, 14 Oct 2013 22:50:10 +0200 > Vladimir 'φ-coder/phcoder' Serbinenko <phco...@gmail.com> пишет: > >> On 14.10.2013 22:45, Chris Murphy wrote: >>> >>> On Oct 14, 2013, at 1:29 PM, Vladimir 'φ-coder/phcoder' Serbinenko >>> <phco...@gmail.com> wrote: >>> >>>>> So it seems that GRUB is using relative pathnames to the default >>>>> subvolume. >>>> This is not intentional. When this part of code was written there was no >>>> set-default available at all so this couldn't be tested and I simply >>>> followed the specification. It told to take root_tree and >>>> root_dir_objectid from superblock then go to "default" directory. What >>>> of this needs to be changed? Just remove "default" and make it part of >>>> path? We would need to change grub-mkrelpath to match runtime behaviour. >>>> Is there a way to detect that mountinfo gives garbage and somehow get >>>> where the real root points? >>> >>> Here's the response. It seems similar but not identical to what you >>> described above. >>> >>> http://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg27955.html >>> >> I don't see what is the difference between that and what GRUB currently >> does. Will look tomorrow morning. > > > I have a feeling that there was some misunderstanding about the > problem. Or btrfs developers do not really intend to have multiple > "active filesystems" at the same time and see "set-default" as the only > legal way to switch between filesystem instances. There are a number of use cases, and already for a while Fedora and openSuse leverage it, where boot, root, and home, are each subvolumes and each is mounted at the proper mount points. In effect three file systems are mounted at the same time. A possible use of set-default would be as follows, each of the items is a subvolume: [top level 5] home ID 274 Fedora 19 ID 259 boot ID 260 root ID 261 home ID 262 Fedora 20 ID 270 boot ID 271 root ID 272 home ID 273 openSuse ID 265 boot ID 266 home ID 267 usr ID 268 var ID 269 By merely changing the set-default subvolume from 259 to 270, I'm booting a different version of Fedora. The same principle can work for making a subvolume "Fedora19-snapshot<date>" and then snapshotting the subvolumes in the Fedora 19 subvolume. Now I can use set-default to rollback to the snapshotted version. This works because at least Fedora's kernel command line, and fstab use relative subvolume paths. So by changing the default subvolume, the relative paths are valid for a different version/snapshot of Fedora. And actually it works because of the unintended behavior I've found, where what looks like an absolute path for prefix, is relative. If it were followed absolutely regardless of default subvolume, prefix= would always point to the exact same grub folder and hence the same grub.cfg. So actually, this is a bit more than somewhat messy, whether the prefix should be formatted relative to the default subvolume, or absolute from top level 5 subvolume. It depends on the use case. So just like '-o subvol=root' has a different meaning than '-o subvol=/root' there is a difference between prefix=(hd0,msdos1)boot/grub/ and prefix=(hd0,msdos1)/boot/grub/ Chris Murphy _______________________________________________ Grub-devel mailing list Grub-devel@gnu.org https://lists.gnu.org/mailman/listinfo/grub-devel
