On 02/25/2017 06:55 PM, Wols Lists wrote: > On 25/02/17 23:41, Phil Turmel wrote: >>> Is there a sound technical reason not to go there, or is it simply a >>>> case of "learn another tool for that job"? The less tools I have to know >>>> the better, imho. > >> Um, no, imnsho. Learn new tools when you need them. > > I don't have a problem with that. All too often people use the tool > they're familiar with when it's the wrong tool. But there's a reason > they do that - it's a familiar tool! >> >> Linux raid has no formal mechanism to cleanly separate a mirror from a >> running array, access it as a backup, and not risk corruption when >> re-attaching it to the array. Most filesystems write to the partition >> when mounting, even for read-only mounts. You cannot safely access the >> disconnected member except via pure block reads. > > 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while disconnected, and the array begins accumulating write-intent bits indicating where any disconnected device is out of date, the array has no way to know what writes are happening to that member.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. > 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 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. Phil -- 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
