Re: Creating an md with 3TB drives.

[David Brown <david@westcontrol.com>]

On 28/09/2011 17:09, Marcin M. Jessa wrote:
> On 9/28/11 10:43 AM, David Brown wrote:
>> On 28/09/2011 10:31, Marcin M. Jessa wrote:
>>> On 9/28/11 10:23 AM, NeilBrown wrote:
>>> [...]
>>>
>>>>>
>>>>> Then I used fdisk to change the partition type to Linux raid auto
>>>>
>>>> You can do that if you like but it will have no effect. The "raid auto"
>>>> partition type only means anything on MBR partitions. You don't need
>>>> to, so
>>>> don't bother.
>>>
>>> Thanks Neil.
>>> Not using fdisk worked like a charm.
>>>
>>
>> Using fdisk probably resulted in the disks going over to MBR rather than
>> GPT, and thus you have a 2TB limit.
>>
>> By the way, you should probably try the parted again and fix the
>> alignment issue (rather than just "ignoring" it) - it will make a /big/
>> difference.
>
> In terms of speed?
> This RAID array is temporarily just so I can back up/restore my broken
> 5x2TB RAID6 but it would be interesting to know what to do.
> I was running
> # parted -a optimal /dev/sdd
> which was suppose to fix that warning but it didn't.
>

Yes, I was thinking in terms of speed.  I haven't gone beyond 2 TB disks 
with MBR myself, so I can't comment on whether your partitions really 
are misaligned, or if it is just an incorrect warning.  But if these 
disks are like most 2 TB disks with 4K sectors but which lie and claim 
to have 512 byte sectors (for compatibility with ancient and inferior 
OS'es), then partition misalignment will be major performance hit.  It's 
worth double-checking that you've got it right here, before you go to 
far with the array.