On Tue, Dec 18, 2012 at 6:13 AM, Hugo Mills [off-list ref] wrote:

On Tue, Dec 18, 2012 at 01:20:20PM +0200, Brendan Hide wrote:

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On 2012/12/17 06:23 PM, Hugo Mills wrote:

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On Mon, Dec 17, 2012 at 04:51:33PM +0100, Sebastien Luttringer wrote:

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Hello,

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I get the feeling that RAID1 only allow one disk removing. Which is more
a RAID5 feature.

   The RAID-1 support in btrfs makes exactly two copies of each item
of data, so you can lose at most one disk from the array safely. Lose
any more, and you're likely to have lost data, as you've found out.

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I'm afraid Btrfs raid1 will not be working before the end of the world.

   It does work (as you demonstrated with the first disk being
removed) -- but just not as you thought it should. Now, you can argue
that "RAID-1" isn't a good name to use here, but there's no good name
in RAID terminology to describe what we actually have here.

Technically, btrfs's "RAID1" implementation is much closer to RAID1E
than traditional RAID1. See
http://en.wikipedia.org/wiki/Non-standard_RAID_levels#RAID_1E or http://pic.dhe.ibm.com/infocenter/director/v5r2/index.jsp?topic=/serveraid_9.00/fqy0_craid1e.html

Perhaps a new name, as with ZFS, might be appropriate. RAID-Z and
RAID-Z2, for example, could not adequately be described by any
existing RAID terminology and, technically, RAID-Z still isn't a
RAID in the classical sense.

   Yeah, we did have a naming scheme proposed, with combinations of
nCmSpP, where n is the number of copies held, m the number of stripes,
and p the number of parity stripes. So btrfs RAID-1 is 2C, RAID-5 on 5
disks would be 4S1P, and RAID-10 on 4 disks would be 2C2S.

...yes.  something like this is not only reflects reality better,, and
actually transfers information in consistent way (vs RAID-XYZ...
meaningless ENUM!) you could maybe do something like:

2C2S : -1S : 0

...or similar, showing:

{normal}
{OFFSET max degraded [rel boundary]}
{OFFSET current}

... which instantly makes the useful boundaries known, along with the
active "panic level" i should be experiencing :)

I'd prefer
to see that than some non-"standard" RAID-18KTHXBYE formulation.

^^^ this. the term "RAID" conjures expectations that run afoul of
btrfs's reality and should thus simply be avoided altogether

IMO, unless you wish/must explicitly correlate some similarity X,
there is no need to even mention the work RAID, because it carries no
information.

-- 

C Anthony