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 |
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