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Raid 1 Performance

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September 18, 2009 11:38:46 PM

The answer to the question of RAID 1's performance seems to very drastically varied among various technology websites. Some say that RAID 1's performance is more than that of using a single disk. Quoting from Wikipedia, it says: "Increased read performance occurs when using a multi-threaded operating system that supports split seeks, as well as a very small performance reduction when writing." So, according to this, theoretically RAID 1 could have gains of up to twice the performance.

Others say that RAID 1 gives absolutely no performance benefit.

Please give me your input.

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a c 415 G Storage
September 19, 2009 3:27:35 AM

The problem is that there are several aspects to hard drive performance, and technology geeks (like economists and statisticians) seize on one aspect or another so they can prove that their point of view is "right".

With RAID 1, you obviously have two complete copies of the data on two different spindles. So at the hardware level, when you read data you can read it from one disk or the other disk. For a single read, it really doesn't make any difference at all, but where you CAN get better performance is if you need to do more than one read at the same time.

If the OS is smart enough to issue multiple requests to the same drive, and the RAID controller is smart enough to be able to handle them, and if your workload is doing enough simultaneous reads to the drive, you can get up to twice the number of I/Os per second. So in performance terms, you can achieve "double the performance".

But note that this is very much like a multi-core CPU - each core is no better than an individual CPU. A multi-core CPU won't run one single-threaded program any faster than a unicore CPU. It takes two programs or a program smart enough to use multiple threads to get the benefit.

In the same way, a RAID 1 volume only gives you benefit when you have multiple programs trying to read the drive, or a single program that's smart enough to issue multiple I/O requests at the same time.

RAID 1 works great in servers where you can have multiple users pounding on the same disk, but it's generally rather less of a benefit in desktops. You'll typically see some modest gains in boot times, but on the whole not a lot more unless you're running some particular program that can take advantage of it or you often run two or more disk-intensive programs at once.
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September 19, 2009 12:02:45 PM

sminlal said:
The problem is that there are several aspects to hard drive performance, and technology geeks (like economists and statisticians) seize on one aspect or another so they can prove that their point of view is "right".

With RAID 1, you obviously have two complete copies of the data on two different spindles. So at the hardware level, when you read data you can read it from one disk or the other disk. For a single read, it really doesn't make any difference at all, but where you CAN get better performance is if you need to do more than one read at the same time.

If the OS is smart enough to issue multiple requests to the same drive, and the RAID controller is smart enough to be able to handle them, and if your workload is doing enough simultaneous reads to the drive, you can get up to twice the number of I/Os per second. So in performance terms, you can achieve "double the performance".

But note that this is very much like a multi-core CPU - each core is no better than an individual CPU. A multi-core CPU won't run one single-threaded program any faster than a unicore CPU. It takes two programs or a program smart enough to use multiple threads to get the benefit.

In the same way, a RAID 1 volume only gives you benefit when you have multiple programs trying to read the drive, or a single program that's smart enough to issue multiple I/O requests at the same time.

RAID 1 works great in servers where you can have multiple users pounding on the same disk, but it's generally rather less of a benefit in desktops. You'll typically see some modest gains in boot times, but on the whole not a lot more unless you're running some particular program that can take advantage of it or you often run two or more disk-intensive programs at once.


well you answer my question of how raid 1 reads.

I was going to ask if raid 1 reads like:

1. raid 0

2. works independently to bring up multiple apps at once

3. work with both 1st hypothesis and 2nd hypothesis. Just depending on amount of apps/size of program(s).

Well from what you put raid 1 works under my 3rd hypothesis.
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a c 415 G Storage
September 19, 2009 6:51:01 PM

warmon6 said:

I was going to ask if raid 1 reads like:
1. raid 0
2. works independently to bring up multiple apps at once
3. work with both 1st hypothesis and 2nd hypothesis. Just depending on amount of apps/size of program(s).
There's no simple answer because it depends entirely on the kind of reading you're doing and on how smart the controller is. For example, let's say you're reading one large file. A RAID0 system can give you high transfer rates because it can be reading pieces of it off two discs at the same time. RAID1 can do the same thing if the controller is smart enough, but a lot of them aren't or are optimized more toward random I/O than large sequential I/Os. This is where looking at benchmarks is important.

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September 19, 2009 8:37:14 PM

sminlal said:
There's no simple answer because it depends entirely on the kind of reading you're doing and on how smart the controller is. For example, let's say you're reading one large file. A RAID0 system can give you high transfer rates because it can be reading pieces of it off two discs at the same time. RAID1 can do the same thing if the controller is smart enough, but a lot of them aren't or are optimized more toward random I/O than large sequential I/Os. This is where looking at benchmarks is important.


i see.
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