Re: [BUG] non-metadata arrays cannot use more than 27 component devices

[Wols Lists <antlists@youngman.org.uk>]

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