Onboard RAID and installing

[Gowator]

My onboard RAID isn’t working.

Onboard RAID controllers and Linux.

The reason for writing this is because of the confusion caused by the recent on-board RAID controllers and people moving to Linux.

This is very general but most onboard RAID controllers are not REAL hardware RAID controllers.

 

This might seem strange but compare them to a WinPrinter or WinModem. They contain the very basics but they leave out a part which is then offset to the OS.

So just like a WinModem may appear to be a real modem so a WinRaid device appears as a real HW solution.

 

A true hardware RAID device is transparent. It may need a driver like a SCSI card needs a driver and it may have software to control it but it should be ‘self-contained’ as it were. Most importantly it should be capable of being swapped from one system to another and only the drivers needed. This is NOT the case for many on board RAIDS.

 

Linux has the facility to do software RAID by itself. Setting this up is beyond this short FAQ, especially since it’s documented in the Linux RAID how to (see LDP) and possible from the Mandrake Control Centre.

 

In brief these onboard RAID ‘controllers’ often offer 0 or 1 or 0+1

 

RAID 0=striping

They are more useful in Windows because the windows file system differentiates drives and partitions and no overall file system exists.

In Linux (*nix) ‘/’ can be composed of many physical disks or partitions and even network nfs or smb mounts etc.

/home might be a different partition, a different disk or even on another computer. Its completely transparent to a user.

The notation ‘../home/myname/Documents’ is valid from anywhere regardless of whether /home is on the same disk/partition/machine or even continent.

 

Windows uses the C: D: E: paradigm. If ‘My Documents’ is on a different drive then it can only be referenced as E:/My Documents etc.

Thus RAID controllers using RAID 0 (striping) are more useful in Windows because they can make physical drives appear as one.

 

The only way in which RAID 0 offers an advantage is that in linux any single file MUST reside in a single partition. That is if your home partition is 10GB and you have a 11GB file onehugemother.avi this can’t be placed in /home.

 

RAID 1=mirroring

Mirroring is just that. A copy is kept on two partitions. The net result is you loose 50% of disk space. As IDE disks get cheaper every year it’s becoming less important about loosing 50% and more important to preserve data, hence RAID 1 is becoming more popular.

 

The problem with software RAID in general is that the RAID metaindex is needed to rebuild the data. RAID 1 gets round this by being so simple. In the event of one disk failing a complete copy exists and the RAID driver rebuilds this onto the other disk. This is less simple in RAID 5.

 

What is important to understand is that the metaindex is software dependant. So far as I know the linux ‘md’ drivers are simple and any software capable of reading the underlying filesystem should be able to read a copy of the data. This includes BSD’s and other *nix’s. Since I don’t know Windows and given the repeated problems it appears the various Windows drivers for onboard semi HW/SW RAID devices doesn’t work this way. That is its OS dependant.

This should be a surprise. All windows filesystems are OS dependent. The fact we can read them from linux/BSD etc. is due to efforts of opensource developers.

 

Why no opensource drivers for these devices then:

Well a developer with a specific need might write them but it’s doubtful. They only duplicate the inherence Linux and *nix ability for RAID but in an OS dependent way. Also they are not guaranteed to be the same across different chipsets/mobo’s.

 

 

So what should I do?

Well think before you use them. I know its tempting, you got a new MOBO and the CD for the RAID drivers is included. One aspect to consider is if you change mobo later you just locked your data into a mobo.

 

If you REALLY need RAID and want to dual boot then a FULL HARDWARE RAID controller is the most reliable. If you NEED RAId then reliability is the number one driver!

 

[moved from Installing Mandrake by spinynorman]

[Gowator]

Blatently just copied from Gentoo 64 install notes

Difference Between Hardware And Software RAID

 

 

 

A Hardware raid controller is always an add-on card, it never comes distributed on a motherboard. It has a bios which you can enter before booting into any OS and usually supports 0,1,1+0,and 5 at a minimum. It has a full CPU onboard that does all raid calculations and I/O, and displays itself to the OS as configured by the raid controller (i.e. if you configure a single raid 5, from 3 drives, it will show up by the OS as one big drive). A Hardware RAID will always be faster than a software raid, and consumes MUCH less CPU time. A hardware RAID controller can come optionally with DIMM slots for caching, and possibly a battery backup for that cache. Hardware raid also limits the possible complexity of a OS driver because the raid functionality is performed exclusively in hardware.

 

 

A Software raid controller can be found in both add-on cards, and is distributed on many motherboards. A software controller may or may not have a BIOS, but the actual raid functionality is actually implemented by the driver in the OS. For this reason, you will NEVER find a software raid controller that can support a bootable RAID5. The OS will be able to see each drive as a standard hard drive, as it is not masked/transformed by the controller in any way. On 2.4 kernels, there was a module that could read many of the SATA controller's BIOSes, set up a linux software raid as specified by that bios, and create a psuedo device was accessible just like a hardware raid would be presented. This 'ataraid' module has not been ported to 2.6, and the 2.4 version does not support SATA controllers, only old PATA software raid controllers. To put it bluntly, a Software RAID controller is nothing more than a standard SATA/PATA controller with possibly a bios to store configuration information, this makes them extremely cheap to manufacture and is why you see them included on many motherboards.