Re: Balance RAID10 with odd device count

[Hugo Mills <hugo@carfax.org.uk>]

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On Mon, Feb 20, 2012 at 08:59:05PM -0500, Tom Cameron wrote:
> Gareth,
> 
> I would completely agree. I only use the RAID vernacular here because,
> well, it's the unfortunate defacto standard way to talk about data
> protection.
> 
> I'd go a step beyond saying dupe or dupe + stripe, because future
> modifications could conceivably see the addition of multiple
> duplicated sets. The case of 4 disks in a BTRFS filesystem with dupe
> running across all of them would be a clear extension I could see. So
> that would be something like 4D. I'm not real sure what you'd use for
> the terminology, but something completely different than RAID-like
> terms is almost certainly best. Just look at the ZFS documentation to
> see how carefully they have to spell out what RAID-Z, Z2, and Z3 do
> because they used the RAID acronym.

   /me opens a plate to put the can of worms on.

   Some time ago, I proposed the following scheme:

<n>C<m>S<p>P

   where n is the number of copies (suffixed by C), m is the number of
stripes for that data (suffixed by S), and p is the number of parity
blocks (suffixed by P). Values of zero are omitted.

   So btrfs's RAID-1 would be 2C, RAID-0 would be 1CnS, RAID-5 would
be 1CnS1P, and RAID-6 would be 1CnS2P. DUP would need a special
indicator to show that it wasn't redundant in the face of a whole-disk
failure: 2CN

   Hugo.

> On Mon, Feb 20, 2012 at 8:47 PM, Gareth Pye <gareth@cerberos.id.au> wrote:
> > On Tue, Feb 21, 2012 at 12:07 PM, Tom Cameron <tomc603@gmail.com> wrote:
> >>
> >> It seems from the BTRFS documentation that the RAID1 profile is
> >> actually "mirror", or store 2 copies of the object. Perhaps when
> >> Oracle makes BTRFS a production option they should more clearly spell
> >> that out.
> >
> >
> > I'd really like BTRFS to not use RAID level terminology anywhere (other than
> > maybe in parenthesis along the lines of: "this is similar to RAIDX") and use
> > less ambigious options as the recommended way to talk about things. As there
> > is good reason to talk about Dup and RAID1 differently as they aren't the
> > same on more than 2 drives. Doing it that way will make people understand
> > what is going on more often, which should be good.
> >
> > It also makes things much easier to remember. Like how much data can you fit
> > on a 6 drive RAID10? I dunno, but I can more intuitively answer that same
> > question when it is phrased as just simply 'dup', or maybe 'dup + stripe'.
> >
> > Is there a difference in BTRFS between dup and raid10?
> >

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=== Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
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