On Fri, Jan 26, 2018 at 7:02 AM, Christophe Yayon <cyayon-l...@nbux.org> wrote:
> 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. No because it's going to open a window where a delayed member drive will mean the volume is mounted degraded, which will happen silently. And current behavior in such a case, any new writes go to single chunks. Again it's silent. When the delayed drive appears, it's not going to be added, the volume is still treated as degraded. And even when you remount to bring them all together in a normal mount, Btrfs will not automatically sync the drives, so you will still have some single chunk writes on one drive not the other. So you have a window of time where there can be data loss if a real failure occurs, and you need degraded mounting. Further, right now Btrfs will only do one degraded rw mount, and you *must* fix that degradedness before it is umounted or else you will only ever be able to mount it again ro. There are unmerged patches to work around this, so you'd need to commit to building your own kernel. I can't see any way of reliably using Btrfs in production for the described use case otherwise. You can't depend on getting the delayed or replacement drive restored, and the volume made healthy again, because ostensibly the whole point of the setup is having good uptime and you won't have that assurance unless you carry these patches. Also note that there are two kinds of degraded writes. a.) drive was missing at mount time, and volume is mounted degraded, for raid1 volumes you get single chunks written; to sync once the missing drive appears you do a btrfs balance -dconvert=raid1,soft -mconvert=raid1,soft which should be fairly fast; b.) if the drive goes missing after a normal mount, Btrfs continues to write out raid1 chunks; to sync once the missing drive appears you have to do a full scrub or balance of the entire volume there's no shortcut. Anyway, for the described use case I think you're better off with mdadm or LVM raid1 or raid10, and then format with Btrfs and DUP metadata (default mkfs) in which case you get full error detection and metadata error detection and correction, as well as the uptime you want. -- Chris Murphy -- 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
