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

[Robin Hill <robin@robinhill.me.uk>]

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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).

Cheers,
    Robin
-- 
     ___        
    ( ' }     |       Robin Hill        <robin@robinhill.me.uk> |
   / / )      | Little Jim says ....                            |
  // !!       |      "He fallen in de water !!"                 |

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