Re: About minimal device number for RAID5/6

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

[-- Attachment #1: Type: text/plain, Size: 1556 bytes --]

On Mon, Aug 15, 2016 at 10:32:25PM +0800, Anand Jain wrote:
> 
> 
> On 08/15/2016 10:10 PM, Austin S. Hemmelgarn wrote:
> >On 2016-08-15 10:08, Anand Jain wrote:
> >>
> >>
> >>>>IMHO it's better to warn user about 2 devices RAID5 or 3 devices RAID6.
> >>>>
> >>>>Any comment is welcomed.
> >>>>
> >>>Based on looking at the code, we do in fact support 2/3 devices for
> >>>raid5/6 respectively.
> >>>
> >>>Personally, I agree that we should warn when trying to do this, but I
> >>>absolutely don't think we should stop it from happening.
> >>
> >>
> >> How does 2 disks RAID5 work ?
> >One disk is your data, the other is your parity.
> 
> 
> >In essence, it works
> >like a really computationally expensive version of RAID1 with 2 disks,
> >which is why it's considered a degenerate configuration.
> 
>    How do you generate parity with only one data ?

   For plain parity calculations, parity is the value p which solves
the expression:

x_1 XOR x_2 XOR ... XOR x_n XOR p = 0

for corresponding bits in the n data volumes. With one data volume,
n=1, and hence p = x_1.

   What's the problem? :)

   Hugo.

> -Anand
> 
> 
> > Three disks in
> >RAID6 is similar, but has a slight advantage at the moment in BTRFS
> >because it's the only way to configure three disks so you can lose two
> >and not lose any data as we have no support for higher order replication
> >than 2 copies yet.

-- 
Hugo Mills             | I always felt that as a C programmer, I was becoming
hugo@... carfax.org.uk | typecast.
http://carfax.org.uk/  |
PGP: E2AB1DE4          |

[-- Attachment #2: Digital signature --]
[-- Type: application/pgp-signature, Size: 836 bytes --]