Re: 64-btrfs.rules and degraded boot

[Chris Murphy <lists@colorremedies.com>]

On Wed, Jul 6, 2016 at 12:24 PM, Andrei Borzenkov <arvidjaar@gmail.com> wrote:
> On Wed, Jul 6, 2016 at 8:19 PM, Chris Murphy <lists@colorremedies.com> wrote:
>>
>> I'm mainly concerned with rootfs. And I'm mainly concerned with a very
>> simple 2 disk raid1. With a simple user opt in using
>> rootflags=degraded, it should be possible to boot the system. Right
>> now it's not possible. Maybe just deleting 64-btrfs.rules would fix
>> this problem, I haven't tried it.
>>
>
> While deleting this rule will fix your specific degraded 2 disk raid 1
> it will break non-degraded multi-device filesystem. Logic currently
> implemented by systemd assumes that mount is called after
> prerequisites have been fulfilled. Deleting this rule will call mount
> as soon as the very first device is seen; such filesystem is obviously
> not mountable.

Seems like we need more granularity by btrfs ioctl for device ready,
e.g. some way to indicate:

0 all devices ready
1 devices not ready (don't even try to mount)
2 minimum devices ready (degraded mount possible)


Btrfs multiple device single and raid0 only return code 0 or 1. Where
raid 1, 5, 6 could return code 2. The systemd default policy for code
2 could be to wait some amount of time to see if state goes to 0. At
the timeout, try to mount anyway. If rootflags=degraded, it mounts. If
not, mount fails, and we get a dracut prompt.

That's better behavior than now.

> Equivalent of this rule is required under systemd and desired in
> general to avoid polling. On systemd list I outlined possible
> alternative implementation as systemd service instead of really
> hackish udev rule.

I'll go read it there. Thanks.


-- 
Chris Murphy