Re: Balance RAID10 with odd device count

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

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On Mon, Feb 20, 2012 at 08:13:43PM -0500, Tom Cameron wrote:
> On Mon, Feb 20, 2012 at 8:07 PM, Hugo Mills <hugo@carfax.org.uk> wrote:
> >
> >   However, you can remove any one drive, and your data is fine, which
> > is what btrfs's RAID-1 guarantee is. I understand that there will be
> > additional features coming along Real Soon Now (possibly at the same
> > time that RAID-5 and -6 are integrated) which will allow the selection
> > of larger numbers of copies.
> >
> 
> Is there a projected timeframe for RAID5/6? I understand it's
> currently not the development focus of the BTRFS team, and most
> organizations want performance over capacity making RAID10 the clear
> choice. But, there are still some situations where RAID6 is better
> suited (large pools of archive storage).

   Rumour has it that it's the next major thing after btrfsck is out
of the door. I don't know how accurate that is. I'm just some bloke on
the Internet. :)

> Also, do we know if the RAID5/6 implementation will simply break data
> into two data objects and one or two parity objects, or will it work
> with an arbitrary number of devices? Meaning, if I have a RAID6 pool
> of 12 drives, will I get 10 data objects and two parity objects?

   AFAIK, the original implementation looked something like the RAID-0
code, so if you have n drives with space for the next block group,
it'll take all n drives to use for the block group. Parity is then
allocated out of those n (with the distribution of the parity blocks
across different drives, as RAID-5 and -6 should do).

   So, allocating a RAID-6 block group of width 1G on your example
12-drive machine, you will indeed end up with 10G of space in that
block group, and 2G of parity data spread across all 12 drives.

   I don't know if the code that will be delivered will allow you to
set a smaller fixed-size stripe width (e.g. 4 data + 2 parity over 8
drives). If the 3-copies RAID-1 code rumour is also true, I would hope
so. Again, I'm just some bloke on the Internet...

   Hugo.

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