Re: [PATCH 0/4] 3- and 4- copy RAID1

[Andrei Borzenkov <arvidjaar@gmail.com>]

18.07.2018 22:42, Goffredo Baroncelli пишет:
> On 07/18/2018 09:20 AM, Duncan wrote:
>> Goffredo Baroncelli posted on Wed, 18 Jul 2018 07:59:52 +0200 as
>> excerpted:
>>
>>> On 07/17/2018 11:12 PM, Duncan wrote:
>>>> Goffredo Baroncelli posted on Mon, 16 Jul 2018 20:29:46 +0200 as
>>>> excerpted:
>>>>
>>>>> On 07/15/2018 04:37 PM, waxhead wrote:
>>>>
>>>>> Striping and mirroring/pairing are orthogonal properties; mirror and
>>>>> parity are mutually exclusive.
>>>>
>>>> I can't agree.  I don't know whether you meant that in the global
>>>> sense,
>>>> or purely in the btrfs context (which I suspect), but either way I
>>>> can't agree.
>>>>
>>>> In the pure btrfs context, while striping and mirroring/pairing are
>>>> orthogonal today, Hugo's whole point was that btrfs is theoretically
>>>> flexible enough to allow both together and the feature may at some
>>>> point be added, so it makes sense to have a layout notation format
>>>> flexible enough to allow it as well.
>>>
>>> When I say orthogonal, It means that these can be combined: i.e. you can
>>> have - striping (RAID0)
>>> - parity  (?)
>>> - striping + parity  (e.g. RAID5/6)
>>> - mirroring  (RAID1)
>>> - mirroring + striping  (RAID10)
>>>
>>> However you can't have mirroring+parity; this means that a notation
>>> where both 'C' ( = number of copy) and 'P' ( = number of parities) is
>>> too verbose.
>>
>> Yes, you can have mirroring+parity, conceptually it's simply raid5/6 on 
>> top of mirroring or mirroring on top of raid5/6, much as raid10 is 
>> conceptually just raid0 on top of raid1, and raid01 is conceptually raid1 
>> on top of raid0.  
> And what about raid 615156156 (raid 6 on top of raid 1 on top of raid 5 on top of....) ???
> 
> Seriously, of course you can combine a lot of different profile; however the only ones that make sense are the ones above.

RAID50 (striping across RAID5) is common.