Sign in with
Sign up | Sign in
RAID 1.5 With IDE: Added Value Or Eyewash?
By ,
1. RAID 1.5 With Two Hard Drives: Added Value Or A Marketing Gag?

More and more people are beginning to prefer a RAID array to a simple hard drive. Private users are switching to take advantage of significantly higher transfer rates. Professional users are trying to protect themselves against any drive defects.

RAID controllers are popular, practical add-ons that motherboard producers like to slap onto their products. In the simplest of cases, you'll get an additional IDE controller for hooking up even more drives. If you like to lean on your computer, you can create your own RAID array from two or four hard drives.

In fact, two RAID modes are found in many home systems and in simple server systems: RAID 0, known as striping, raises the data transfer rate; RAID 1, also called mirroring, permanently synchronizes (mirrors) the contents of one hard drive with its twin, effectively halving your overall storage capacity.

There's no doubt that RAID 0 is the fastest method and RAID 1 the safest. You will really experience the benefits of RAID 1 if your hard drive fails - the system will continue functioning and you can return your RAID to normal functioning once the defective drive has been replaced. RAID 0, on the other hand, throws security overboard in favor of capacity and peformance: the more drives you have, the more likely a defective drive will cause your system to crash.

But surely there must be a RAID mode that provides data security and higher performance than a single hard drive - right? Sure - it's called RAID 5. This RAID mode distributes data and parity information over no less than three hard drives. It requires a powerful controller, though, because you need a small RISC chip to calculate the parity data (XOR). A cheaper alternative is RAID 01 or 10, which works with four hard drives, and mirrors two RAID 0 arrays (0+1, or vice versa: 1+0).

While RAID 5 is not available as an onboard solution, RAID 01 and 10 are.

The costs, however, are still mighty high. The best thing for a small system would be somewhere between RAID 1 and RAID 5. And that is precisely what DFI and HighPoint have come up with. According to the manual for the latest LANParty series mainboards, the HPT372N can also handle RAID 1.5 with two hard drives. We decided to give it a spin.

2. HighPoint HPT372N

We found this controller on two motherboards made by DFI. The LANParty 875 Pro and LANParty KT400A models both have the following chip:

DFI has adorned the IDE RAID controller with an image-laden sticker...

... that conceals familiar technology: a HighPoint HPT372.

Now that we've seen what's underneath, we can put the sticker back in place.

The HPT372 is HighPoint's standard model, as it were. While its big brother, the HPT374, can now handle RAID 5 (without XOR engine) under the current firmware release, HighPoint has all of a sudden decided to transfer its amassed experience in storing parity data to this simpler controller chip. The result is called RAID 1.5. This is the first time that we have ever seen this kind of configuration deployed with only two drives.

3. RAID 15 And RAID 1.5 In Detail

First things first: HighPoint does not operate "true" RAID 15. As its name indicates, a RAID 15 array is made up of several logical drives, each of which is comprised of RAID 1 arrays. This is called a "nested" RAID setup:

RAID 15 is made up of at least three logical elements (the minimum requirement for RAID 5) that are in turn comprised of RAID 1 arrays. RAID 51 is exactly the opposite: it mirrors two RAID 5 arrays.

It's not difficult to see that combining two RAID modes greatly enhances data security. With a RAID 15 array, one drive can crash in each RAID 1 block without bringing the whole system to its knees.

So HighPoint should be aiming for something similar if it wants to hawk its product with the RAID 15 label. The manual for DFI's LANParty 875 Pro and LANParty KT400A motherboards contains the following table:

This makes it abundantly clear that RAID 1.5 offers optimum performance and data security (oval on the right) - without having anything to do with real RAID 15. So far, so good. What doesn't quite cut it, though, is the claim that it supports both striping and mirroring. The answer to this question is simply the dot between the 1 and the 5, as we are talking about RAID 1.5. Both RAID 1.5 and RAID 15 does nothing else than combining striping (read access over two drives simultaneously) and mirroring (data is written like in RAID 1).

In fact, although the controller does handle physical striping - i.e., data are alternately written (or read) to one disk and then the other, maximizing the data stream because both drives are being used - that's still not enough: it would basically be nothing more than plain old RAID 0. The capacity available with the RAID 1.5 array equals the capacity of a single hard drive, leaving the other half of the total capacity for parity data."

4. RAID 1.5 From HighPoint: Striping With Parity Data

In theory, the read operation should be almost as quick as they are with a RAID 0. Running in some kind of striping mode, both hard drives are read from simultaneously.

Writing should not give us any improvement, as data still has to be written onto both drives in order to read in a sripe-mode.

In the worst-case scenario, a hard drive gives up the ghost while the system would still run with the second (imaged) drive. The only disadvantage is that all read data has to be taken from only one drive now.

Basically, this method should be as quick at sequentially reading as RAID 0, while writing should be equally fast than with RAID 1.

5. Test Setup
Test System
Processor AMD Athlon XP 2200+
Motherboard DFI LANParty KT400A
VIA KT400A Chipset
BIOS : May 6, 2003
RAM 256 MB DDR333/PC2700, CL2 Corsair Micro
Controller & Hard Drives VIA VT8235 UltraATA/133
HighPoint HPT372N On-Board
BIOS 2.342
2x Western Digital WD2000BB
Display Adapter nVIDIA GeForce3 Ti4200, 64 MB
Network Card 3COM 905TX PCI 100 MBit
Operating System Windows 2000 Professional
5.00.2195, Service Pack 3
Benchmarks and Tests
Performance Measurements HD Tach 2.61, c’t h2benchw
I/O performance Intel IOMeter
Drivers and Settings
Graphics Driver NVIDIA Reference Driver 41.09
Drivers Intel Application Accelerator 2.3
DirectX Version 9.0
Resolution 1024 x 768, 16 Bit, 85 Hz refresh

Owing to time restraints, we limited the capacity of the RAID array to 25 GB. Tests on total capacities of up to 400 GB would have taken much longer and would not have produced a different result. All the low-level measurements (IOMeter, HDTach) are only run without partitions anyway and refer to the total capacity of the RAID array.

6. Benchmark Results

Data Transfer Charts

RAID 0


RAID 1


RAID 1.5


WD2000BB, single


7. Data Transfer Performance

Access Time

8. Application Performance: Fileserver

RAID 1.5 produces a few advantages for mid-range query depths in the fileserver application model. Only when 64 or more sequential blocks are called (typical sequential reading) is RAID 0 able to play its trumps.

Application Performance: Webserver

The webserver profile also shows advantages for RAID 1.5 - this time as clear as day!

CPU Utilization

9. Conclusion: Pros And Cons For RAID 1.5 Balance Out

Unfortunately, the RAID controllers on DFI's motherboards won't let you reach the holy grail of optimal performance and perfect data security with only two hard drives. RAID 1.5 can't dispel the disadvantages of having redundant data storage with RAID 1: on the one hand, there's the small usable net capacity; on the other, there's no sign of any real gain in performance.

In theory, RAID 1.5 is a better performer than RAID 1 because it can read from two hard drives, just because you access the data using the same zipper-type method as with RAID 0 (striping). While we could not detect higher transfer rates with RAID 1.5, CPU utilization was certainly lower and it outperformed in the fileserver and webserver benchmarks that required good I/O performance. The main reason for this good showing is that RAID 1.5 requires a lower CPU utilization. Additional positive side effects of RAID 1.5 are somewhat shorter access times.

A serious disadvantage compared to RAID 1, however, is the fact that (as with RAID 0) you can't read any drive separately with a standard controller. The data stocks can be restored all the same because of the parity information on the hard drive. You'll need to have HighPoint's HPT372N for the restoration, though, since no other controller can handle RAID 1.5 - and if it can, then only in the nested RAID setup we mentioned above (combination of two RAID modes).

Finally, the benefits for home use are interesting, but are they strong enough to justify creating a RAID array based on HighPoint/ DFI's RAID 1.5? Overall performance is only slightly improved, while data security actually increases by redundant storage onto two hard drives. The main reason for having "secure" RAID is, after all, optimized failure safety and/ or data security - and this is where RAID 1 is equally good and supported by a larger variety of controllers. Power users are best served, as before, with RAID 0, while genuine security coupled with higher performance is still associated with higher costs and, consequently, with RAID 10 or RAID 5.

RAID 1.5 might still be an attractive option in the lower server segment (webservers), though, as the test results prove that the test system with RAID 1.5 is actually better than a simple RAID 0 in terms of I/O performance. We'll have a comprehensive test in late summer that will show to what extent the IDE RAID controller will be able to take on the SCSI-dominated segment.