Re: Confusion with setting up new RAID6 with mdadm

[Zoltan Szecsei <zoltans@geograph.co.za>]

On 2010-11-15 21:53, Neil Brown wrote:
> On Mon, 15 Nov 2010 20:01:48 +0200
> Zoltan Szecsei<zoltans@geograph.co.za>  wrote:
>
>    
>> Hi,
>> One last quick question:
>> ****************************************************
>> I have compiled the following mdadm on my Ubuntu 64 bit 10.04 Desktop
>> system:
>> root@gs0:/home/geograph# uname -a
>> Linux gs0 2.6.32-25-generic #45-Ubuntu SMP Sat Oct 16 19:52:42 UTC 2010
>> x86_64 GNU/Linux
>> root@gs0:/home/geograph# mdadm -V
>> mdadm - v3.1.4 - 31st August 2010
>> root@gs0:/home/geograph#
>>
>> ****************************************************
>> I have deleted the partitions on all 8 drives, and done a mdadm -Ss
>>
>> root@gs0:/home/geograph# fdisk -lu
>>
>> Disk /dev/sda: 2000.4 GB, 2000398934016 bytes
>> 255 heads, 63 sectors/track, 243201 cylinders, total 3907029168 sectors
>> Units = sectors of 1 * 512 = 512 bytes
>> Sector size (logical/physical): 512 bytes / 512 bytes
>> I/O size (minimum/optimal): 512 bytes / 512 bytes
>> Disk identifier: 0x00000000
>>
>> Disk /dev/sda doesn't contain a valid partition table
>>
>> Disk /dev/sdb: 2000.4 GB, 2000398934016 bytes
>>
>> ******************************************************
>> Based on the above "assume-clean" comment, plus all the help you guys
>> have offered, I have just run:
>> mdadm --create /dev/md0 --metadata=1.2 --auto=md --assume-clean
>> --bitmap=internal --bitmap-chunk=131072 --chunk=512 --level=6
>> --raid-devices=8 /dev/sd[abcdefgh]
>>
>> It took a nano-second to complete!
>>
>> The man-pages for assume-clean say that "the array pre-existed". Surely
>> as I have erased the HDs, and now have no partitions on them, this is
>> not true?
>> Do I need to re-run the above mdadm command, or is it safe to proceed
>> with LVM then mkfs ext4?
>>      
> It is safe to proceed.
>    

Too cool (A for away at last :-) )
Neil: Big thanks to you and the others on this list for all the patience 
& help you guys have given.,
Kind regards,
Zoltan
> The situation is that the two parity block are probably not correct on most
> (or even any) stripes.  But you have no live data on them to protect, so it
> doesn't really matter.
>
> With the current implementation of RAID6, every time you write, the correct
> parity blocks are computed and written.  So any live data that is written
> will be accompanies by correct parity blocks to protect it.
>
> This does *not* apply to RAID5 as it sometimes uses the old parity block to
> compute the new parity block.  If the old was wrong, the new will be wrong
> too.
>
> It is conceivable that one day we might change the raid6 code to perform
> similar updates if it ever turns out to be faster to do it that way, but it
> seems unlikely at the moment.
>
> NeilBrown
>
>
>    




-- 

===========================================
Zoltan Szecsei PrGISc [PGP0031]
Geograph (Pty) Ltd.
P.O. Box 7, Muizenberg 7950, South Africa.

65 Main Road, Muizenberg 7945
Western Cape, South Africa.

34° 6'16.35"S 18°28'5.62"E

Tel: +27-21-7884897  Mobile: +27-83-6004028
Fax: +27-86-6115323     www.geograph.co.za
===========================================



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