On Mon, Jun 25, 2012 at 10:46:01AM -0700, H. Peter Anvin wrote:

On 06/25/2012 08:21 AM, Chris Mason wrote:

quoted

Yes and no.  If you have 2 drives and you add one more, we can make it
do all new chunks over 3 drives.  But, turning the existing double
mirror chunks into a triple mirror requires a balance.

-chris

So trigger one.  This is the exact analogue to the resync pass that is
required in classic RAID after adding new media.

   You'd have to cancel and restart if a second new disk was added
while the first balance was ongoing. Fortunately, this isn't a problem
these days.

   Also, it occurs to me that I should just check -- are you aware
that the btrfs implementation of RAID-1 makes no guarantees about the
location of any given piece of data? i.e. if I have a piece of data
stored at block X on disk 1, it's not guaranteed to be stored at block
X on disks 2, 3, 4, ... I'm not sure if this is important to you, but
it's a significant difference between the btrfs implementation of
RAID-1 and the MD implementation.

   Hugo.

-- 
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