Re: Confusion with setting up new RAID6 with mdadm

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

On 2010-11-14 21:50, Luca Berra wrote:
> On Sun, Nov 14, 2010 at 05:36:38PM +0200, Zoltan Szecsei wrote:
>>
>> *Partition alignment?
>> Is this relevant for modern HDs (I'm using 5900rpm Seagate 2TB drives)
> for modern hdds with 4k sectors it is
> new fdisk and/or parted should already know how to align
fdisk reports 512b sectors:
root@gs0:/home/geograph# fdisk -lu

WARNING: GPT (GUID Partition Table) detected on '/dev/sda'! The util 
fdisk doesn't support GPT. Use GNU Parted.

Disk /dev/sda: 2000.4 GB, 2000398934016 bytes
256 heads, 63 sectors/track, 242251 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

    Device Boot      Start         End      Blocks   Id  System
/dev/sda1              63  3907024127  1953512032+  fd  Linux raid 
autodetect

(All 8 disks are as above)


> in any case, since you want to use the whole space for raid, why create
> partitions at all, md works nicely without
OK

>
>> *Chunk size:
>> After reading MANY different opinions, I'm guessing staying at the 
>> default chunk size is optimal? Anyone want to add to this argument?
> i believe the default in newer mdadm is fine
>
>>
>> *After partitioning the 8 drives, is this the correct sequence?
>> mdadm --create /dev/md0 --metadata=1.2 --auto=md --chunk=64 
>> --level=raid6 
> why you state the chunk size here, i tought you wanted to stay with the
> defauklt
because on my mdadm, 64 is the default - and I was just re-enforcing 
that for the reader.
root@gs0:/home/geograph# mdadm -V
mdadm - v2.6.7.1 - 15th October 2008
root@gs0:/home/geograph#
>> *After this, do I mkfs ext4 first, or LVM first?
> if you want to use lvm it would be lvm first, but... do you want to?
> there is no point if the aim is allocating the whole space to a single
> filesystem.
Because I might want to join this array to another one at a later stage 
- I would then have 2 boxes each with 8 drives, and each with a SiI3132 
card on the same motherboard.
I might use the second box to mirror the first, or to extend it - not 
sure of my needs yet.


>
>> *What stride and stripe values should I use?
> new toolstack should already find the correct stripe/stride for you
How would I check ?

root@gs0:/home/geograph# mkfs.ext4 -V
mke2fs 1.41.11 (14-Mar-2010)
         Using EXT2FS Library version 1.41.11
root@gs0:/home/geograph#
>
> one more note:
> for such a big array i would suggest to create a bitmap, so in case of
> an unclean shutdown you do not have to wait for 27 hours for it to
> rebuild. an internal bitmap will do.
>
Nice tip - I'll look into it - Thanks,
Zoltan


-- 

===========================================
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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