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Re: Supermicro SAS controller



On Sun, 06 May 2012 11:46:21 +0000, Ramon Hofer wrote:

> On Sat, 05 May 2012 11:15:22 +0000, Camaleón wrote:

>>> On the other hand it isn't possible to have different disk sizes in a
>>> raid 6 neither.
>> 
>> I think yes, that you can, but only the lowest of the disk capacities
>> will be used (this applies to all of the RAID levels). Software RAID
>> has not such limitaion because you can mirror partitions, instead.
> 
> Ok so with sw raid I can use partitions as devices. This means I could
> divide my drives into 500 GB pieces and like this use the whole size of
> the disks?

If your hard disk capacity is ~1.5 TiB then you can get 3 partitions from 
there of ~500 GiB of size (e.g., sda1, sda2 and sda3). For a second disk, 
the same (e.g., sdb1, sdb2 and sdb3) and so on... or you can make smaller 
partitions. I would just care about the whole RAID volume size.

> Wouldn't it be easier to have e.g. four of each size and put them into
> raid5?

With software raid you have more choices because you can partition as you 
like: you can use the whole disk capacity or make small chunks and use 
them to be part of a RAID volume.

Again, RAID5 is not something advisable and mdadm also supports RAID 6 :-)

>>> So my plan seems still reasonable to me to have several 4 disks raid 5
>>> arrays. Like that I'm flexible to add bigger disks in future as they
>>> become cheaper and still can keep my old 1.5 TB disks. And if I would
>>> go for raid 6 with the 4 disk array I would loose a third of the
>>> capacity.
>> 
>> (...)
>> 
>> You've been warned :-)
> 
> Yes and I appreciate that!
> But I can't see any other solution without loosing 500 GB of the two TB
> disks :-?

When using the whole hard disk capacity for the array:

- A RAID 5 volume with x4 1.5 TiB disks will give you an available space 
of 4.5 TiB (the sum of the number of the disks minus 1 drive).

- A RAID 6 volume with x4 1.5 TiB disks will give you an available space 
of 3 TiB (the sum of the number of the disks minus 2 drives). 

That's the price for the added data security. If you are constrained 
about hard disk space, remember that you can add LVM and your spacing 
problems are be solved >;-)

Greetings,

-- 
Camaleón


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