BRTFS in /var/lib/mythtv

[21:33] <danilin> kernel 3.19.I have a  btrfs multi device filesystem: a /dev/sda5 on a msdos formatted SSD hard disk AND a /dev/sdb on a pendrive GPT formatted. My problem is with /etc/fstab, options  device=/dev/sda5,device=/dev/sdb is th e only way I can make it work at reboot. UUID,PARTUUiD,dev=/dev/disk/by-id , anything I have tried make it impossible to reboot. I'm using mythbuntu 14,04,03. any help,idea? I was thinking to reinstall everything in SSD hard disk GPT,not msdos.
[21:33] <darkling> You need to run btrfs dev scan before mounting the filesystem.
[21:34] <darkling> If / is on that filesystem, you need an initramfs to do that from.
[21:35] <darkling> If it's not the / filesystem, then installing the btrfs-progs package for your distribution (btrfs-tools on Debian/Ubuntu) should install the relevant hooks in udev to do the scan as each device is enumerated.
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[21:38] <danilin> well, it's working well when I put in /etc/fstab as options device=/dev/sda5,/device=/dev/sdb. but /dev/sdb is a pendrive, so if I move it it can become /dev/sdc, so I was trying to use another way to name it in /etc/fstab
[21:39] <darkling> You've found the exact drawback of using device=
[21:39] <darkling> The only way of solving this is to run btrfs dev scan.
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[21:44] <danilin> I suppose my distro run alone "btrfs dev scan" during boot, but It doesn't resolve the problem if /etc/fstab options are not as I said before. I have read here https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices about "All these options can also be set from the kernel command line, through root=/fstype=/rootflags=. "
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[21:45] <danilin> may you explain better what does it mean "All these options can also be set from the kernel command line, through root=/fstype=/rootflags=. "
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[21:46] <darkling> That's somewhat mangled.
[21:47] <darkling> What it's saying is that if you're trying to mount a filesystem as / and want to pass mount parameters for that, you use rootflags=<parameters> on the kernel command line in your bootloader to pass them, instead of putting them in /etc/fstab.
[21:48] <darkling> However, that still doesn't deal with the issue that your device names can change.
[21:48] <darkling> Is this filesystem your root filesystem or not?
[21:48] <danilin> not,it's not
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[21:48] <darkling> OK, so the rootflags= advice is irrelevant.
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[21:49] <darkling> You just have to ensure that btrfs dev scan is run, with all the devices present, before the FS is mounted.
[21:50] <darkling> Most distributions will do this automatically through udev if their packaged btrfs-progs is installed.
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[21:52] <darkling> After the scan is run, you don't need to pass device= parameters to mount, and you'll be able to use LABEL= or UUID= in the first column of fstab to identify the filesystem to mount.
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[22:00] <danilin> Ok, mythbuntu 14.04.03 with 3.19 kernel , if I remove "device=" in /etc/fstab  mythbuntu is unable to boot "an error occured while mounting /var/lib/mythtv" , udev is installed, btrfs-tool is installed. M to go manual, S to skip. If I manually mount it ,"mount /var/lib/mythtv" it works.
[22:02] <darkling> In which case, it sounds like it's not installing appropriate hooks in the udev configuration.
[22:04] <darkling> Either that, or it's not triggering the udev rules before trying to mount.
[22:05] <darkling> I suppose it could be attempting to mount before all the devices have been scanned.
[22:05] <darkling> I think you probably need to be having this conversation with the mythTV people.
[22:05] <danilin> thanks for your help, a last question "where did I find btrfs command??" I can't find it /usr/bin and in /usr/sbin. I was thinking to add "btrsfs dev scan" when booting the xfce session.
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[22:06] <darkling> It'll depend on the distribution, but one of /usr/bin, /usr/sbin, /bin or /sbin would be most likely.
[22:06] <danilin> ok thanks really
[22:06] <darkling> Or just use "shich btrfs"
[22:07] <darkling> Err.. "which btrfs"