Re: Supermicro SAS controller
On Sun, 06 May 2012 12:18:33 +0000, Camaleón wrote:
> 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.
Sorry I don't get it.
Let's assume I have 4x 1.5 TB and 4x 2 TB. I divide each drive into 500
GB partitions. So three per 1.5 TB and four per 2 TB disk. Then I put the
28 partitions (4x3 + 4x4) in a raid 6?
>> 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 >;-)
My problem is that I don't have much experience with raid. Only about the
two years where I only had one drive failure which was false alarm. I
could put it right back in.
So I think I'll have to burn my fingers myself to understand the little
(or maybe even misleading in some sense) security of raid5...
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