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Date:      Fri, 7 Jun 2013 11:18:50 -0500
From:      Dan Nelson <dnelson@allantgroup.com>
To:        Quartz <quartz@sneakertech.com>
Cc:        FreeBSD questions <freebsd-questions@freebsd.org>
Subject:   Re: ZFS: raid VS copies=n
Message-ID:  <20130607161850.GA37520@dan.emsphone.com>
In-Reply-To: <51B201D4.3000705@sneakertech.com>
References:  <51B201D4.3000705@sneakertech.com>

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In the last episode (Jun 07), Quartz said:
> How does the ZFS option 'copies=n' and raid relate to and interact with
> each other?  specifically recovery in the event of a failure.  For
> example, is having three disks in a raid-1 configuration with copies=1
> effectively the same as having three disks in a raid-0 with copies=3?  Are
> the copies distributed uniformly across all drives in the pool, or
> concentrated, or what?  What happens with configs like a raid-z2 with
> copies=2?  Which / how many disks can you lose?

The code will try to place the extra copies on different vdevs, but if
that's not possible, it will try and place them at least 1/8th of the disk
size apart on the same disk.  Copies aren't meant to protect against whole
disk loss, but more local damage within a disk.

https://blogs.oracle.com/bill/entry/ditto_blocks_the_amazing_tape
https://blogs.oracle.com/relling/entry/zfs_copies_and_data_protection

-- 
	Dan Nelson
	dnelson@allantgroup.com



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