Re: RAID1 storage server won't boot with one disk missing

[Goffredo Baroncelli <kreijack@libero.it>]

Hi Anand,


On 2015-09-17 17:18, Anand Jain wrote:
>  it looks like -o degraded is going to be a very obvious feature,
>  I have plans of making it a default feature, and provide -o
>  nodegraded feature instead. Thanks for comments if any.
> 
> Thanks, Anand

I am not sure if there is a "good" default for this kind of problem; there are several aspects:

- remote machine:
for a remote machine, I think that the root filesystem should be mounted anyway. For a secondary filesystem (home ?), may be that an user intervention could be better (but without home, how an user could log?).

- spare:
in case of a degraded filesystem, the system could insert a spare disk; or a reshaping could be started (raid5->raid1, raid6->raid5)

- initramfs:
this is the most complicated things: currently most initramfs don't mount the filesystem if all the volumes aren't available. Allowing a degraded root filesystem means:
	a) wait for the disks until a timeout
	b) if the timeout expires, mount in degraded mode (inserting a spare disk if available ?)
	c) otherwise mount the filesystem as usual

- degraded:
I think that there are different level of degraded. For example, in case of raid6 a missing device could be acceptable; however in case of a raid5, this should be not allowed; and an user intervention may be preferred.


In the past I suggested the use of an helper, mount.btrfs [1], which could handle all these cases better without a kernel intervention:
- wait for the devices to appear
- verifying if all the needed devices are present 
- mounting the filesystem passing
     - all the devices to the kernel (without relying to udev and btrfs dev scan...)
     - allowing the degraded mode or not                     (policy)
     - starting an insertion of the spare                    (policy)

G.Baroncelli

[1] http://www.spinics.net/lists/linux-btrfs/msg39706.html


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