Re: About minimal device number for RAID5/6

[Anand Jain <anand.jain@oracle.com>]

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 ?

-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.