RAID 1

Mirroring seems to be one of the most widely spread type of RAID arrays. Even its software implementation doesn't take a lot of CPU resources. Since everything you need in this case is to duplicate the commands for the second HDD so that to have two absolutely identical hard drives. But as usual, things are not so simple as they might seem at first sight…

In case of software implementation of RAID 1, then there will be twice as many commands and data transferred via IDE bus during writes, which can reduce the average write speed. However, using UDMA66/100 controllers may help to solve this problem.

If there is a special controller used to create RAID 1 array, then it appears possible to send all the commands just once, and the controller will duplicate them. And if the controller has its own cache, it will allow unloading the bus significantly, since the controller will take the data to be written onto the second HDD from its own cache and not from the system memory.

Also there is one more feature, which seems to be implemented by 3ware guys in their controller card. If the data should be written first on the Master drive and only after the process is entirely completed on the Slave drive, then the reading can be carried out from any of them, since the info written on both is identical. In other words, we can request the HDDs in turns when reading, so that the whole thing looks very much like RAID 0 with that only difference that the array doesn't grow bigger.

3ware Escalade 6400

Now let's see how 3ware's patented TwinStor technology works:

It is worth mentioning that Drive 1.0 didn't detect any difference in performance of RAID 1 array compared with a single HDD (allegedly in RAID 0). The only difference was a bit smaller access time by RAID 1 array. Nothing indicated even a slight increase in the transfer rate.

And what about HDTach?

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Here the tests also didn't show anything special about RAID 1 implementation on 3ware Escalade 6400…

But let's not give way to despair. We still have two more testing sets, WinBench99 and Intel IOMeter ahead:

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We would like to draw your attention to the transfer rate shown by RAID 1: it is much higher than in case of a single HDD, but lower than in case of RAID 0 for 2 HDDs.

The way this linear read graph looks signals that both HDDs do work in parallel, though not so efficient as in RAID 0 array, anyway. :-)

The same conclusion can be drawn from the table below. As you see, RAID 1 almost always appears between RAID 0 for 1 HDD (if you remember we decided to regard a single HDD working with a controller card as a kind of RAID 0 array) and RAID 0 for 2 HDDs.

And in NTFS the results turned out even more intriguing. The single HDD dashed forward, so that its leadership is beyond all doubts, while RAID 1 showed the worst performance here.

As we have already found out when testing the controllers in RAID 0 arrays, they are not suitable for good results in WinBench. That's why you shouldn't be surprised with what you see here now.

Well, we wonder if TwinStor technology will have any effect on Intel IOMeter results:

We decided to simplify our investigation by offering you some graphs, which are a bit easier to perceive than numbers and tables. First let's compare the controller's performance in RAID 1 with that shown in RAID 0 for 1 and 2 HDDs:

Hm… RAID 1 seems to be brave enough to compete with RAID 0 for 2 drives, having left the single drive far behind.

Well, almost the same picture as in the previous case. The difference between these patterns lies only with the requests size, which doesn't tell at all on the overall performance, as we can see.

In DataBase pattern, RAID 1 results appeared a bit closer to those of a single HDD. Everything corresponds completely to the theory, because this pattern includes more writes (33%) than Fileserver and WorkStation (20%) and the advantages of TwinStor appear evident only for reads. Going on with the idea, we can state that in RandomWrite pattern with 100% of write operations RAID 1 will show absolutely the same performance as a single hard disk drive:

However, there is nothing even close to what we expected to see! RAID 1 graph is always above RAID 0 for 1 HDD, which means that there is one more factor, which we haven't taken into consideration… As for a sharp increase, which you can see on the interval between 64 and 256 I/Os, you should bear in mind that our scale along the x-axis is a bit incorrect and in reality the increase is not so severe.

Well, these results seem quite logical: the bus gets slightly overloaded in case of too many requests submitted simultaneously.

Summing up we would like to make a very brief conclusion: TwinStor technology is worth it!

Adaptec AAA-UDMA

Adaptec didn't implement any specific technologies for RAID 1, however, we will do our best to find something out:

Well, nothing special so far. Let's pass over to HDTach:

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Again, there isn't anything extraordinary here. The average access time got a bit lower and the CPU utilization got a bit higher. Moreover, we could also point out the slowing down of all write speeds (min, max and average).

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Here RAID 1 is always slower than a single hard drive (except High-End FrontPage only). However, the linear read graphs are just impeccable.

The situation didn't change in a different file system: everything is just as in the previous case.

Now goes Intel IOMeter:

Let's compare RAID 1 and RAID 0 the same way we have just done it:

Well, we can't help asking here:" what's the advantage of RAID 0?" To tell the truth, the controller seems not to care at all how many HDDs are connected to it and what kind of RAID configuration it is working in…

See above for comments :-)

Only a growing number of writes managed to make RAID 0 for 2 hard drives prove faster.

This graph gives a perfect idea of a classical approach to RAID-building. A little advantage of a single HDD over RAID 1 configuration during high workloads is probably connected with the fact the controller itself gets less loaded in this case.

And here the picture looks really interesting: any configuration other than that with a single hard disk drive appears slower than the latter.

All in all, mirroring on Adaptec AAA-UDMA is none other but simple mirroring.

Promise SuperTrak100

As usual, the first thing to come is Drive 1.0:

The results here are absolutely inadequate… We think it is the last time we resorted to this test for RAID controllers testing.

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Judging by the Winbench 99, we can say that RAID 1 by Promise SuperTrak100 turned out very diverse. In some cases, it appears faster than a single drive, in other cases it appears faster than RAID 0 even. But still there are situations when it is an absolute failure.

Well, three different controllers bore three different graphs. We would like to draw your attention to the fact that RAID 1 graph starts far above all the others and the higher gets the workload, the closer it moves to the single HDD graph. It seems to us that Promise controller doesn't single out the Master and Slave drives in the Mirroring configuration, but simply duplicated the commands for both of them. And since the average access time is lower in RAID 1, the graph starts higher that the others.

The graph here is similar to the previous one reflecting the patterns similarity.

The more writes the pattern includes, the smaller gets the gap between RAID 1 and RAID 0 in the lower workload area.

In RandomWrite pattern the fact that HDDs in RAID 0 work in parallel tells on the performance and allows RAID 0 to surpass RAID 1 configuration quite tangibly.

Unfortunately, the controller bus is unable to get wider that's why duplicating all commands cuts down its practical width. And the writing on two hard disk drives appears faster than on a single hard disk drive only if the disk drive is that particular bottleneck that slows down the whole process.

Controllers Comparison

Well, let's begin with WinBench 99. Fortunately, there were only two HDDs involved in this test that is why we managed to slightly reduce the number of graphs here.

In all the tests except High-End Disk Winmark for FAT32, the lead belongs to Adaptec AAA-UDMA controller.

And as for IOMeter, we will have to consider each pattern separately, since each of them models a possible application field for the controller and features its particular specifics.

Well, once again we start thinking that TwinStor technology definitely makes sense.

Well, it definitely makes much more sense than we have initially though.

Here again a bit more sense can be added. Just look what performance gain shows 3ware controller at the maximum load. Very simple architecture and such an impressive result! Maybe the engineers should think about it…

Here the leadership was taken by Promise SuperTrak100.

In the last test, all the controllers could have shown better results if it were not for their limited bandwidth.

Summing up this time we will based our verdict mostly on IOMeter results. And judging by them, 3ware controller deserves all the praise.