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-l...@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. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
