Hi all, I've been reading up on mdadm and RAID's in linux, but I can't seem to figure out whether what I want to do is possibe or not. I have a 30 GB IDE drive just for the OS, so this RAID will be purely data - it does NOT need to be bootable. Here's what I'd like to set up:

Hard Drives:
400 GB IDE
250 GB IDE
200 GB IDE
120 GB IDE
500 GB SATA
500 GB SATA

Controllers:
SIIG SATA RAID card
Promise Ultra 100TX2

I'd like to stripe the two 500 GB SATA drives using the hardware RAID controller to create one large volume that will be 1000 GB (easy enough, and already done). Then I'd like to create a software RAID that combines the 4 IDE drives into one large volume that will be 970 GB. Then I'd like to mirror the two volumes, leaving me with one volume that is approx. 970 GB.

I think I can find drivers for the two controllers, they seem to be pretty standard. The big uncertainty is whether linux software RAID can do spanning (or whatever the proper term is), and whether you can mirror two "virtual" volumes that are, themselves, RAIDed.

Also, is there anything else I should be aware of, or any other complications that I should be prepared for?

Thanks!

Yes, it's possible. What you're wanting is a logical volume, and you can set up an LV using any disks/volumes you'd like.

However, with any software RAID setups, you're going to pay a price. They're much slower than any hardware RAID systems, and are going to eat up system CPU and memory.

Before you get too excited, I'm pretty sure that neither of those cards is an actual "hardware RAID controller". That doesn't mean that you can't use them in an array, of course.

Each IDE hard drive should be on its own channel. If two drives are on one channel and one hard drive fails, the channel will come with it that also includes another hard drive, so two hard drives will come down. How Promise controllers work is they use one channel for each of their two channels, so all hard drives will fail when one channel fails. Promise controllers are really not reliable in Linux because of this design flaw. I suggest buy two Highpoint Rocket133 because they have two hardware channels.

Yes, Quakeboy02 is correct that those cards are software RAID not hardware RAID. A hardware RAID controller is like 3ware controllers.

I suggest using EVMS because it is better than LVM2. Probably better just add all those drives to either LVM2 or EVMS with out putting them in RAID. Both LVM2 and EVMS will try to stripe data. The problem will be backing up the data.

Two 500 GB hard drives in RAID-0 will not give you 1 TB because how manufactures advertise capacity in base 10 (our number system). Base 10 to base 2 (computer number system) loses a lot of information. A terabyte in computer terms is 1024 GB and 1000 GB is how we interpret a TB. If we convert our 1000 GB into computer number system, it will be about 953 GB which is 71 GB shy of a real TB. Your 970 GB that you thought will be around 925 GB.

You could setup a linear RAID that includes 400 GB,250 GB,200 GB,120 GB. Then setup the two 500 GB in RAID-0. Next use RAID-1 to mirror the linear array with the RAID-0 array at a sacrifice of not using the other 25 GB. This will be very, very complex setup to keep track and maintain for each boot up.

In the case of a business system, I would agree that he should re-examine his hardware. But, it seems to be the case that he has a bunch of legacy hardware that he wants to take advantage of in a home system, so I'm not so sure this advice is applicable.

If it were up to me for a home theatre type of data server, I'd just setup the two 500GB SATA drives in a striped mdadm array, and leave it alone. If dual booting, then use dmraid instead of mdadm. If it filled up, then I'd consider adding the IDE drives. But, really, as long as it's just DVDs or MP3s being served, it doesn't really matter how you put them together; other than for the vanity points of using a RAID.

Thanks for all the replies, everyone!

Quakeboy, you are correct that this is NOT a business server, just a home file server for non-critical data. I would be very upset if I lost it all though, which is why I'd like some sort of mirroring.

I am fully aware that two 500 GB drives do not a 1000 GB volume make - I was just rounding up for convenience.

The SATA controller is actually a hardware RAID, and of course my long-term intention is to eventually buy two more 500 GB SATA drives and use those to replace the odd assortment of random drives making up the other volume. Then I'd have four 500 GB SATA drives and a simpler setup.

Anyway, for the moment I don't care about availability, but I do want redundancy because some of the original drives (the odd sized ones) are starting to get a little old. I currently have about 600 GB of data, and I need room to grow that over the next few months.

So using dmadm I can create a Logical Volume that combines the 4 odd-size IDE drives, then mirror it to the hardware raided sata drives.

Thanks again!

mcd, you are incorrect. The controllers that you listed are software RAID. Use Linux software RAID instead of using software RAID from the controller.

Either LVM2 or EVMS will be better for you because you can expand when you need to.

I still recommend using another brand in place of the Promise controller. Promise controllers may work well in Windows, but in Linux they do not work well. The controller that I suggest earlier is cheap.

maybe i'm confused then. when i turn on the computer, the BIOS loads and checks the memory. the very next thing that happens is that the SATA card detects the drives and displays a message saying press F4 to enter RAID setup, or something similar. then the motherboard IDE controllers detect the drives connected to them, then the Promise card detects the drives connected to it, and then finally the OS boots.

when i press F4 to enter RAID setup (immediately after the BIOS does it's memory check, before the onboard IDE drives are recognized) i can create new raid sets, delete existing raid sets, etc.

inside the OS, as long as i have the proper driver installed i see the raid set as a physical disk. i can format it, partition it, mount it, etc. just as i do a normal drive.

i thought that indicated a hardware raid. what is the definition of hardware raid vs. software raid? am i missing something?

mcd

PS. i know the promise card sucks, and has no hardware raid features. it is just something i had laying around, and is only intended to get me through the next few months until i can get a couple more large sata drives.

SIIG does not make hardware RAID controllers. They make software RAID controllers. The difference is very hard to tell. Hardware RAID controllers contains several chips such as a controller chip, processor, memory, XOR, and any miscellaneous chips that may improve performance for multiple queues. Hardware RAID controllers are sold over 100 dollars. Software RAID controllers are sold very, very cheaply, so they could be sold at a dollar store. Price could be one way to tell. The inner workings of software RAID controllers works by copying the RAID software to the main computer's memory. The main processor executes this special data to handle all RAID tasks plus controller tasks. Windows will see the software RAID as a RAID and display the array as one physical drive. In Linux, it is a different story. It will see all the drives that is attached to the controller with out noticing the software RAID that is in the physical memory. In order for Linux to see this software RAID, you need to use dmraid utilities to see the RAID as one drive. The dmraid utilities is completely separate from mdadm utilities. An OS will see hardware RAID array as one drive with out ever have to do any processing. If you have to dual boot and you want the array to be seen in any OS, hardware RAID controllers will be the best.

Anybody can convert a standard controller into software RAID if it has sufficient space in its flash or BIOS chip. I could add RAID support to my Abit KA7-100, but I do not want to. My Abit TH7II-RAID has RAID support but I do not want to use it. Both motherboards uses the same Highpoint chip and model which is HPT370.

got it, thanks for the info!

mcd