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

[Andrei Borzenkov <arvidjaar@gmail.com>]

20.07.2018 20:16, Goffredo Baroncelli пишет:
> On 07/20/2018 07:17 AM, Andrei Borzenkov wrote:
>> 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.
> 
> Yeah someone else report that. But other than reducing the number of disk per raid5 (increasing the ration number of disks/number of parity disks), which other advantages has ? 

It allows distributing IO across virtually unlimited number of disks
while confining failure domain to manageable size.

> Limiting the number of disk per raid, in BTRFS would be quite simple to implement in the "chunk allocator"
> 

You mean that currently RAID5 stripe size is equal to number of disks?
Well, I suppose nobody is using btrfs with disk pools of two or three
digits size.