Re: About minimal device number for RAID5/6

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2016-08-15 21:32, Qu Wenruo wrote:
>
>
> At 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.
>
> I totally agree with the fact that 2 disk raid5 is just a slow raid1.
>
>>  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.
>
> It's true that btrfs doesn't support any other raid level which can
> provide 2 parities.
>
> But the use case to gain the ability to lose 2 disks in a 3 disk raid6
> setup seems more like a trick other than normal use case.
It absolutely is a trick, but until we have support for more than 2 
copies in raid1 mode, it is also an absolutely legitimate use case.
>
> Either in mkfs man page, or warning at mkfs time (but still allowing to
> do it), IMHO it's better to tell user "yes, you can do it, but it's not
> a really good idea"
Agreed.