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Date:      Sun, 12 Jun 2016 19:47:17 -0500
From:      Brandon J. Wandersee <brandon.wandersee@gmail.com>
To:        Yuri <yuri@rawbw.com>
Cc:        FreeBSD Questions <freebsd-questions@freebsd.org>
Subject:   Re: ZFS: Is 'zpool add' really irreversible?
Message-ID:  <864m8xewyy.fsf@WorkBox.Home>
In-Reply-To: <22e9b8aa-3171-f399-f3a8-b71eb92210f5@rawbw.com>
References:  <d13abf72-3903-796a-1d47-e3a7b776ebf8@rawbw.com> <86shwiax38.fsf@WorkBox.Home> <22e9b8aa-3171-f399-f3a8-b71eb92210f5@rawbw.com>

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Yuri writes:

> People may reasonably want to remove some disks in some layouts, due
> to failures, etc, and ZFS just lacks the flexibility to do that.

Obviously, if you're dealing with any sort of RAID you'll occasionally
be replacing failed disks; ZFS is no different, and you can swap out
disks while the system is running. You just can't remove a *virtual
device*. You can't just shuffle disks around willy-nilly, because you'd
effectively destroy the storage pool in the process. There's a minimum
number of disks that need to be attached, and that minimum changes as
you add virtual (not necessarily physical) devices to a
pool. Traditional RAID has the same sort of limitation: create a RAID 5
array out of three disks, then remove two disks. You've just destroyed
the array.

If you want to temporarily add a single disk to a system, you can just
create a second pool on it. There's no arbitrary limit to how many pools
a system can have. ZFS has real limitations, but they're not that strict.

-- 

::  Brandon J. Wandersee
::  brandon.wandersee@gmail.com
::  --------------------------------------------------
::  'The best design is as little design as possible.'
::  --- Dieter Rams ----------------------------------



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