large disk > 8 TB

[fluffles.net]

Ivan Voras wrote:
> Barkley Vowk wrote:
>
>   
>> It looks like he created a 32bit disk label. He needs to use either the
>> raw device, or gpt partitions I think.
>>
>> Ie. /dev/mdid1 or /dev/mdid1p1 instead of /dev/mdid1s1
>>     
>
> You're right :)
> I didn't think of checking that - a wrong assumption at my part.
>   

If you are using partitions on a RAID device, you have to make sure you
don't end up with a stripe misalignment. If there is misalignment then
you end up requiring 2 I/O requests whereas otherwise 1 I/O request
would suffice. Naturally this decreases IO performance (less IOps). To
avoid a misalignment you have two options:

- not using partitions, but using the raw device like Barkley said
- use partitions (GPT or normal) and create one large partition, which
starts at offset 1MiB (not MB!) thus 1024*1024 bytes. Note that you
probably need to convert this to sectors (512 bytes).

If you do option 2 right, then the partition will start at precisely the
start of a new stripe block - thus there is no misalignment. You can use
offsets like 64KiB and 128KiB but i prefer to use 1MiB since that will
work with all stripesizes (up to 1MiB, which is rarely used).

Merry christmas to all. :)

Regards,
Veronica