Re: from 2x RAID1 to 1x RAID6 ?

["Stefan G. Weichinger" <lists@xunil.at>]

Am 08.06.2011 16:20, schrieb Phil Turmel:
> Hi All,
> 
> On 06/08/2011 06:33 AM, David Brown wrote:
>> On 08/06/2011 12:11, John Robinson wrote:
>>> On 08/06/2011 10:38, David Brown wrote:
>>>> On 08/06/2011 01:59, Thomas Harold wrote:
>>>>> On 6/7/2011 4:07 PM, Maurice Hilarius wrote:
>>>>>> On 6/7/2011 12:12 PM, Stefan G. Weichinger wrote:
>>>>>>> Greetings, could you please advise me how to proceed?
>>>>>>> 
>>>>>>> On a server I have 2 RAID1-arrays, each consisting of 2
>>>>>>> TB-drives:
>>>>>>> 
>>>>>>> ..
>>>>>>> 
>>>>>>> Now I would like to move things to a more reliable RAID6
>>>>>>> consisting of all the four TB-drives ...
>>>>>>> 
>>>>>>> How to do that with minimum risk?
>>>>>>> 
>>>>>>> .. Maybe I overlook a clever alternative?
>>>>>> 
>>>>>> RAID 10 is as secure, and risk free, and much faster. And
>>>>>> will cause much less CPU load.
>>>>>> 
>>>>> 
>>>>> Well, with both a pair of RAID1 arrays and a pair of RAID-10
>>>>> arrays, you can lose 2 disks without losing data, but only if
>>>>> the right 2 disks fail.
>>>>> 
>>>>> With RAID6, any two of the four can fail without data loss.
>>>>> 
>>>> 
>>>> It /sounds/ like RAID6 is more reliable here because it can
>>>> always survive a second disk failure, while with RAID10 you
>>>> have only a 66% chance of surviving a second disk failure.
>>>> 
>>>> However, how often does a disk fail? What is the chance of a
>>>> random disk failure in a given space of time? And how long will
>>>> it go between one disk failing, and it being replaced and the
>>>> array rebuilt? If you figure out these numbers, you'll have the
>>>> probability of losing your RAID10 array due to the second
>>>> critical disk failing.
>>>> 
>>>> To pick some rough numbers - say you've got low reliability,
>>>> cheap disks with a 500,000 hour MTBF. If it takes you 3 days to
>>>> replace a disk (over the weekend), and 8 hours to rebuild, you
>>>> have a risk period of 80 hours. That gives you a 0.016% chance
>>>> of having the second disk failing. Even if you consider that a
>>>> rebuild is quite stressful on the critical disk, it's not a big
>>>> risk.
>>> 
>>> It's not so much that the mirror disc might fail that I'd be
>>> worried about, it's that you might find the odd sector failure
>>> during the rebuild - this is the reason why RAID5 is now so
>>> disliked, and the reasons apply similarly to RAID1 and RAID10
>>> too, even if you're only relying on one disc ('s worth of data)
>>> being perfect rather than two or more.
>> 
>> I can see that problem, but it again boils down to probabilities.
>> The chances of seeing an unrecoverable read error are very low,
>> just as with other disk errors.
> 
> The chances of any given unrecoverable read error are low, but during
> the rebuild, you are going to read every sector of the remaining
> drive in a mirror pair, or every sector of every remaining drive in a
> degraded raid5.  On large drives, you suddenly have a probability of
> uncorrectable error during rebuild that is orders of magnitude larger
> than the risk of a generic drive failure (in the rebuild window).
> 
> Since Stefan reported that he does backups to this array, I suspect
> the performance is less important than the redundancy.  The
> difference in redundancy is *very* significant.
> 
> Here's some stats on disk failures themselves: 
> http://www.storagemojo.com/2007/02/19/googles-disk-failure-experience/
>
>  Here's some stats on read errors during rebuild: 
> http://storagemojo.com/2010/02/27/does-raid-6-stops-working-in-2019/
> 
> If I recall correctly, Google switched to exclusive use of
> triple-disk mirrors on its production servers for this very reason.
> (I can't find a link at the moment....)
> 
>> The issue with RAID5 is that people often had large arrays with
>> multiple disks, and on a rebuild /every/ sector had to be read.  So
>> if you have a ten disk RAID5 and are rebuilding, you are reading
>> from all other 9 disks - you have 9 times as high a chance of
>> having an unrecoverable read error ruin your day.
>> 
>> I look forward to the day bad block lists and hot replace are ready
>> in mdraid - it will give us close to another disk's worth of
>> redundancy without the cost.  For example, if one half of your
>> raid1 mirror fails but is not totally dead (such as by having too
>> many bad blocks), during rebuild you can keep both the good and bad
>> halves in place.  Then if there is a read failure on the "good"
>> half, you can probably still get the data from the "bad" half.
> 
> I don't see where either of these actually help the "rebuild after
> disk failure" situation?

phew ... thanks to all of you for your statements ...
I have to read through all this at first ...

;-)

Thanks, Stefan