On 26/02/17 00:07, Phil Turmel wrote: >> Because to do so doesn't make sense? Or because nobody's bothered to do >> > it? I get grumpy when people implement corner cases without bothering to >> > implement the logically sensible options - bit like those extremely >> > annoying dialog boxes that give you three choices, "yes", "no", "yes to >> > all". What about no to all? > Because while disconnected, and the array begins accumulating > write-intent bits indicating where any disconnected device is out of > date, the array has no way to know what writes are happening to that > member. And therefore any re-add will introduce unknowable corruptions. > There is no way to control what writes happen to that member, and > drives don't naturally keep a log of writes that have happened. The data to > safely do what you want simply doesn't exist. Your only known safe > choice is to disable write-intent bitmaps, forcing complete resync on > --re-add. Sorry to drag this up again, but where are these write intent bits going to come from? And it's a backup. Why am I going to re-add, unless I'm going to wipe the old backup and create a new one? > >> > I feel like mirror-raid is perfect for doing backups. > Your feelings are wrong. Sorry. LVM is the perfect tool because it > entirely controls the snapshot and doesn't have to re-add it. > I think we're talking at cross-purposes here :-) You're talking about creating a snapshot and backing it up. I'm talking about creating a mirror, which IS the backup. VERY different technique, same end result. And your way is more complicated - more room for sys-admin cock-up :-) >> > I take your point >> > that linux hasn't implemented that feature (particularly well), but >> > surely it's a feature that *should* be there. I know I know - "patches >> > welcome" :-) > Good luck creating the necessary data from thin air. It's not a > question of writing patches. > mdadm --build /dev/mdbackup --device-count 2 /dev/md/home missing ... hotplug sd-big ... madam /dev/mdbackup --add /dev/sd-big ... wait for sync to finish ... mdadm --stop mdbackup ... unplug sd-big ... You've made me think about it deeper than before - thanks - and I can think of at least one potential show-stopper, but write-intent bitmaps and missing raid data are most definitely not it :-) And why do I think my way is "better" (for certain values of "better" :-) - because your way only works if it was planned in advance. My way - if the show stopper isn't - will work on ANY running system whether planned or not. That said, my problem probably is a show stopper :-( Cheers, Wol -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
