Re: degraded permanent mount option

[Christophe Yayon <cyayon-list@nbux.org>]

Hi Austin,

Thanks for your answer. It was my opinion too as the "degraded" seems to be flagged as "Mostly OK" on btrfs wiki status page. I am running Archlinux with recent kernel on all my servers (because of use of btrfs as my main filesystem, i need a recent kernel).

Your idea to add a separate entry in grub.cfg with rootflags=degraded is attractive, i will do this...

Just a last question, i thank that it was necessary to add "degraded" option in grub.cfg AND fstab to allow boot in degraded mode. I am not sure that only grub.cfg is sufficient... 
Yesterday, i have done some test and boot a a system with only 1 of 2 drive in my root raid1 array. No problem with systemd, but i added rootflags and fstab option. I didn't test with only rootflags.

Thanks. 


-- 
  Christophe Yayon
  cyayon-list@nbux.org

On Fri, Jan 26, 2018, at 15:18, Austin S. Hemmelgarn wrote:
> On 2018-01-26 09:02, Christophe Yayon wrote:
> > Hi all,
> > 
> > I don't know if it the right place to ask. Sorry it's not...
> No, it's just fine to ask here.  Questions like this are part of why the 
> mailing list exists.
> > 
> > Just a little question about "degraded" mount option. Is it a good idea to add this option (permanent) in fstab and grub rootflags for raid1/10 array ? Just to allow the system to boot again if a single hdd fail.
> Some people will disagree with me on this, but I would personally 
> suggest not doing this.  I'm of the opinion that running an array 
> degraded for any period of time beyond the bare minimum required to fix 
> it is a bad idea, given that:
> * It's not a widely tested configuration, so you are statistically more 
> likely to run into previously unknown bugs.  Even aside from that, there 
> are probably some edge cases that people have not yet found.
> * There are some issues with older kernel versions trying to access the 
> array after it's been mounted writable and degraded when it's only two 
> devices in raid1 mode.  This in turn is a good example of the above 
> point about not being widely tested, as it took quite a while for this 
> problem to come up on the mailing list.
> * Running degraded is liable to be slower, because the filesystem has to 
> account for the fact that the missing device might reappear at any 
> moment.  This is actually true of any replication system, not just BTRFS.
> * For a 2 device raid1 volume, there is no functional advantage to 
> running degraded with one device compared to converting to just use a 
> single device (this is only true of BTRFS because of the fact that it's 
> trivial to convert things, while for MD and LVM it is extremely 
> complicated to do so online).
> 
> Additionally, adding the `degraded` mount option won't actually let you 
> mount the root filesystem if you're using systemd as an init system, 
> because systemd refuses to mount BTRFS volumes which have devices missing.
> 
> Assuming that the systemd thing isn't an issue for you, I would suggest 
> instead creating a separate GRUB entry with the option set in rootflags. 
>   This will allow you to manually boot the system if the array is 
> degraded, but will make sure you notice during boot (in my case, I don't 
> even do that, but I'm also reasonably used to tweaking kernel parameters 
> from GRUB prior to booting the system that it would end up just wasting 
> space).
> > 
> > Of course, i have some cron jobs to check my array health.
> It's good to hear that you're taking the initiative to monitor things, 
> however this fact doesn't really change my assessment above.