Re: misunderstanding of spare and raid devices? - and one question more

[David Brown <david@westcontrol.com>]

On 01/07/2011 10:50, Robin Hill wrote:
> On Fri Jul 01, 2011 at 09:23:43 +0200, David Brown wrote:
>
>> On 30/06/2011 23:28, NeilBrown wrote:
>>> On Thu, 30 Jun 2011 16:21:57 +0200 Karsten Römke<k.roemke@gmx.de>   wrote:
>>>
>>>> Hi Phil
>>>>>
>>>>> If your CPU has free cycles, I suggest you run raid6 instead of raid5+spare.
>>>>>
>>>>> Phil
>>>>>
>>>> I started the raid 6 array and get:
>>>>
>>>> Personalities : [raid0] [raid1] [raid10] [raid6] [raid5] [raid4]
>>>> md0 : active raid6 sde5[4] sdd5[3] sdc5[2] sdb2[1] sda3[0]
>>>>          13759296 blocks level 6, 64k chunk, algorithm 2 [5/5] [UUUUU]
>>>>          [=================>...]  resync = 87.4% (4013184/4586432) finish=0.4min speed=20180K/sec
>>>                                     ^^^^^^
>>> Note: resync
>>>
>>>>
>>>> when I started the raid 5 array I get
>>>>
>>>> md0 : active raid5 sdd5[4] sde5[5](S) sdc5[2] sdb2[1] sda3[0]
>>>>          13759296 blocks level 5, 64k chunk, algorithm 2 [4/3] [UUU_]
>>>>          [=>...................]  recovery =  6.2% (286656/4586432) finish=0.9min speed=71664K/sec
>>>                                     ^^^^^^^^
>>> Note: recovery.
>>>
>>>>
>>>> so I have to expect a three times less write speed - or is this calculation
>>>> to simple ?
>>>>
>>>
>>> You are comparing two different things, neither of which is write speed.
>>> If you want to measure write speed, you should try writing and measure that.
>>>
>>> When you create a RAID5 mdadm deliberately triggers recovery rather than
>>> resync as it is likely to be faster.  This is why you see a missed device and
>>> an extra spare.  I don't remember why it doesn't with RAID6.
>>>
>>
>> What's the difference between a "resync" and a "recovery"?  Is it that a
>> "resync" will read the whole stripe, check if it is valid, and if it is
>> not it then generates the parity, while a "recovery" will always
>> generate the parity?
>>
>  From the names, recovery would mean that it's reading from N-1 disks,
> and recreating data/parity to rebuild the final disk (as when it
> recovers from a drive failure), whereas resync will be reading from all
> N disks and checking/recreating the parity (as when you're running a
> repair on the array).
>
> The main reason I can see for doing a resync on RAID6 rather than a
> recovery is if the data reconstruction from the Q parity is far slower
> that the construction of the Q parity itself (I've no idea how the
> mathematics works out for this).
>

Well, data reconstruction from Q parity /is/ more demanding than 
constructing the Q parity in the first place (the mathematics is the 
part that I know about).  That's why a two-disk degraded raid6 array is 
significantly slower (or, more accurately, significantly more 
cpu-intensive) than a one-disk degraded raid6 array.

But that doesn't make a difference here - you are rebuilding one or two 
disks, so you have to use the data you've got whether you are doing a 
resync or a recovery.


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