> -----Original Message----- > From: linux-btrfs-owner@xxxxxxxxxxxxxxx [mailto:linux-btrfs- > owner@xxxxxxxxxxxxxxx] On Behalf Of Duncan > Sent: Friday, October 02, 2015 3:12 AM > To: linux-btrfs@xxxxxxxxxxxxxxx > Subject: Re: RAID5 doesn't mount on boot, but you can afterwards? > > Hugo Mills posted on Thu, 01 Oct 2015 17:46:15 +0000 as excerpted: > > > On Thu, Oct 01, 2015 at 07:04:43PM +0200, Sjoerd wrote: > >> On Thursday 01 October 2015 02:21:23 Duncan wrote: > >> > >> > That's very likely because unlike traditional single-device > >> > filesystems (including single-device btrfs), multi-device btrfs has > >> > multiple devices it must know about before it can mount the device, > >> > while mount only feeds it one device. > >> > > >> > There are two ways to tell btrfs (the kernel side) about the other > >> > devices. > >> > > >> > 1) Do a btrfs device scan before trying to mount. > >> > > >> > 2) Name the component devices in the mount options, using the > >> > device= option (multiple times as necessary to list all devices). > >> > > >> Option 2 was to simplest to check and that works. Thanks for the tip! > >> Still weird that my single devide SSD BTRFS bootdisk just worked fine > >> (althought it's using the uuid offcourse)...But it would imply to me > >> that there's a btrfs device scan run before mounting it. > > > > Not really. A single deice FS doesn't need the scan. > > Yes. > > A mount command takes a single device pointer either on the > commandline, or from fstab. For traditional single-device filesystems, that > pointer, whether via traditional /dev/* path, or by (udev-mediated) LABEL=, > UUID=, etc, is all that's needed, one device, and the kernel knows what it > was because it was supplied in that pointer. > > But for non-traditional multi-device filesystems, like btrfs in multi- device > mode (as opposed to btrfs used on only a single device where the single > device pointer works fine), a single device pointer only provides part of the > necessary information, the kernel has to figure out what other devices are > needed by some other method. With btrfs, there are two such other > methods, btrfs device scan, or supplying the other devices via device= > mount option, with as many such device= options used as necessary to list > all filesystem component devices. > > ... Which is what I was trying to explain in the earlier reply as well, when I > specifically included the "including single-device btrfs" > parenthetical in the traditional device class, contrasted with multi- device > btrfs, but apparently that specific bit didn't transfer. > > Well, at least the practical solution, use device scan or name the devices in > mount options, did. =:^) > Beginner here, so just if it helps: My two-device raid 1 mounts on boot in Fedora 22 (uuid in fstab, no further devices specified) but I mount the fs via uuid while Sjoerd mounted subvolumes. From what I understand (not much) it's either an subvolume issue or Fedora must then somehow perform a device scan before handling fstab. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
