Re: Mounting raid without a btrfsctl scan

[Goffredo Baroncelli <kreijack@xxxxxxxxx>]

On Saturday 15 May 2010, Matt Brown wrote:
> Hi,
Hi Matt,
> 
> Would it be possible and feasible to support mounting btrfs
> raid/multi-device filesystems without having to run 'btrfsctl -a'?
> 
> Currently, as you may know, if a one wants to attach a btrfs raid
> filesystem to a system (usb, hotswap, reboot, etc), the user or program
> has to run:
> 
> btrfsctl -a (or similar)
> mount /dev/sdb1 /mount/point
> 
> While this works, it will require patching of various subsystems
> involved with managing disks, such as udev, mkinitrd, dracut, hal, and
> others. Each one will have to know to scan, then mount.

In a my previous post ([RFC] btrfs, udev and btrfs - 16/April 2010), I 
suggested a solution for this kind of problem. I a Debian/Ubuntu system it is 
not necessary to patch anything, it is only required to put some files in the 
initramfs-tool configure directories.

IIRC, also the md (RAID) subsystem require a scan from the user space to find 
and activate the volumes. I think also dm (LVM) subsystem requires the same.

> 
> For example, I have a system that has a btrfs raid1 as root. However, I
> had to patch the boot loader (dracut) so during boot it would scan just
> before mounting the root filesystem.
> 
> I filed a bug with dracut, but the more I think of it, the more it seems
> that either mount.btrfs should be taking care of this, or another part
> of btrfs.

If it would be mount.btrfs to perform the scan, that means a scan for every 
mounting. I think that is better to separate the two function. The scan has to 
be performed a device discovery time, and not a the mounting time.


> 
> Any thoughts or plans on the matter?
> 
> Thanks,
> Matt

BR
Goffredo
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