Re: Supermicro SAS controller
On Sun, 06 May 2012 12:35:40 +0000, Ramon Hofer wrote:
> On Sun, 06 May 2012 12:18:33 +0000, Camaleón wrote:
>> 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.
x4 1.5 TiB → sda, sdb, sdc, sdd
x2 2 TiB → sde, sdf
> I divide each drive into 500 GB partitions. So three per 1.5 TB and
> four per 2 TB disk.
1.5 TiB hard disks:
sda1, sda2, sda3
sdb1, sdb2, sdb3
sdc1, sdc2, sdc3
sdd1, sdd2, sdd3
2 TiB hard disks:
sde1, sde2, sde3, sde4
sdf1, sdf2, sdf3, sdf4
> Then I put the 28 partitions (4x3 + 4x4) in a raid 6?
Then you can pair/mix the partitions as you prefer (when using mdadm/
linux raid, I mean). The "layout" (number of disks) and the "raid level"
is up to you, I don't know what's your main goal.
What I usually do is having a RAID 1 level for holding the operating
system installation and RAID 5 level (my raid controller does not support
raid 6) for holding data. But my numbers are very conservative (this was
a 2005 setup featuring 2x 200 GiB SATA disks in RAID 1 and x4 SATA disks
of 400 GiB. which gives a 1.2 TiB volume).
Yet, despite the ridiculous size of the RAID 5 volume, when the array
goes down it takes up to *half of a business day* to rebuild, that's why
I wanted to note that managing big raid volumes can make things worse :-/
>> 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.
Yeah, false possitives are also a PITA :-), that's why a good (and
usually "costly" hardware raid controller is a must if you want to have a
peaceful journey with your RAID setup.
> 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...
We all get experience by doing things and decide based on our criteria,
there's no other way to learn (errors and success included).
Greetings,
--
Camaleón
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