Re: What RAID type and why?

[Michael Evans <mjevans1983@gmail.com>]

On Wed, Mar 10, 2010 at 9:47 AM, Goswin von Brederlow <goswin-v-b@web.de> wrote:
> Michael Evans <mjevans1983@gmail.com> writes:
>
>> On Sun, Mar 7, 2010 at 4:52 AM, Goswin von Brederlow <goswin-v-b@web.de> wrote:
>>> Neil Brown <neilb@suse.de> writes:
>>>
>>>> On Sat, 06 Mar 2010 18:17:44 -0500
>>>> "Guy Watkins" <linux-raid@watkins-home.com> wrote:
>>>>
>>>>> }
>>>>> } At a minimum I would build a 3-disk raid 6.  raid 6 does a lot of i/o
>>>>> } which may be a problem.
>>>>>
>>>>> If he only needs 3 drives I would recommend RAID1.  Can still loose 2 drives
>>>>> and you don't have the RAID6 I/O overhead.
>>>>>
>>>>
>>>> and as md/raid6 requires at least 4 drives, RAID1 is not just the best
>>>> solution to survive two failures on a 3-device array, it is the only solution.
>>>>
>>>> NeilBrown
>>>
>>> Except that there also is raid10 with 3 mirrors. :)
>>>
>>> MfG
>>>        Goswin
>>>
>>> PS: Why doesn't raid6 still not allow 3 drives for the special case of
>>> converting raid1 -> raid6?
>>>
>>>
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>>
>> That should be obvious:
>>
>> Possible stripes:
>>
>> Start:
>> 1, 1, 1;
>> 2, 2, 2;
>
> Start:
> 1, 1, 1;
> 2, 2, 2;
> 3, 3, 3;
> ...
>
>> 'raid6' overtake...
>> 1, q, Q;
>> 2, q, Q;
>
> Middle:
> 1, P, Q;
> P, Q, 2;
> Q, 3, P;
> ...
>
> End:
> 1, 2, P, Q;
> 4, P, Q, 3;
> P, Q, 5, 6;
> ...
>
>> 'raid6' overtake with missing;
>> 1, (missing 2), q, Q;
>> 3, (missing 4), q, Q;
>>
>> In the first overtake case you have the requirement of generating 200%
>> parity, which probably won't work for the algorithm and is a silly
>> idea in general since it's computationally far less expensive to store
>> another copy of either form of data instead.
>
> The sick 3 disk raid6 case should have both the P and Q identical to the
> data block. It is indeed computational a waste to go through the
> expensive P/Q parity algorithm for the same result as mirroring but this
> is only ment as a transitional state.
>
>> In the second you're gaining the space of a second disk at the cost of
>> being already degraded; why not just go for raid 5 instead?
>>
>> You can overtake raid5 later with raid6 if you add more devices.
>
> Because then you are going from 2 mirror disks to 1 parity disk even if
> only temporary. You are reducing the number of disks failures you can
> survive from 2 to 1 and the high load during a reshape makes a failure
> more likely than normal operations.
>
> Or can you go from 3 way raid1 to 4 disk raid6 in a single step?
>
> MfG
>        Goswin
>

You are not planning on staying with 3 devices though.

Just stick with 2 redundancy raid 1 until you have four devices.
Then overtaking from raid 1 + hotspares at 4 devices total to raid 6
with 2 data devices and 2 parity devices per stripe makes sense.
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