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  Converting RAID 0 to RAID 5: Is it possible? 
 
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Nathan Daniels Nov 02, 2012, 10:13pm EDT Reply - Quote - Report Abuse
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My SSD just died (third on in two years) and I lost my whole OS. While it's being RMA'd, I want to buy two identically-sized ones and set up a RAID 5 array to prevent this from happening again. However, I don't want to wait three weeks to get my desktop back to work so I was thinking I could set the two new ones up in a RAID 0, install Windows and get to work. Once the replacement for the original drive comes in, would it be possible to convert that RAID 0 to a RAID 5? It seems perfectly possible; the effective size is the same. A quick internet search, however, was inconclusive.

I'll be doing the RAID with my motherboard, the ASUS M5A99X EVO.


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"The more we sweat in peace, the less we bleed in war."
-Patton
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Bungle Nov 03, 2012, 08:10pm EDT Reply - Quote - Report Abuse
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>> Re: Converting RAID 0 to RAID 5: Is it possible?
There are some enterprise class RAID controllers that can change the RAID levels of arrays, I have serious doubts that any onboard controllers will have this capability. You're probably going to have to wait for your SSD to get RMA'd or resign yourself to setting up your OS 2x.

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Nathan Daniels Nov 04, 2012, 01:25am EDT Reply - Quote - Report Abuse
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>> Re: Converting RAID 0 to RAID 5: Is it possible?
I was afraid of that. Oh well; it won't kill me to install the OS twice. If it does let me change the RAID level, I'll let you know

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"The more we sweat in peace, the less we bleed in war."
-Patton
Dr. Peaceful Nov 05, 2012, 12:32am EST Reply - Quote - Report Abuse
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Edited: Nov 05, 2012, 01:34am EST

 
>> Re: Converting RAID 0 to RAID 5: Is it possible?
To answer your question, the only way I can think of that may be possible is imaging. For hardware RAID (doesn't matter which type), the OS should see the entire array as a single logical disk. If you make a file-based image (not block-based) of the logical disk, you should be able to restore it even under a different type of RAID setup. I can't say for sure though, until you try it.

Your own thread from month ago, which I happened to also reply. If image works from RAID to non-RAID, the same should apply from one RAID to another. http://www.hardwareanalysis.com/content/topic/78231/

With 3 identical disks, RAID 5 waste 1 disk for parity, leaving you with 2 disks of space. RAID 0 on 2 disks, gives you 2 disks of space. So you have no size mismatch issue here, either, as you already mentioned.

One thing I do want to question is why do you choose RAID 5? RAID 5 is stripe with parity, which distributes stripe blocks and parity data across all drives in the array. The AMD SB950 controller you have doesn't give you much utility for repairing the array in case of a drive failure. Each disk in a RAID 5 is pretty much useless without the controller. In another word, you can't get any useful data out of it by moving it to another PC. RAID 1 is the only RAID that will give you full disk clone, which you can take out and read from another PC, which is more useful. RAID 5 is pretty overkill for a gaming machine really. It doesn't give you much performance gain, since you're already using SSD's, and the controller uses large chunk of resource from Windows since it's not a full hardware RAID! Doesn't give you much data redundancy either, since the onboard controller is limited in functionality.

Nathan Daniels Nov 05, 2012, 09:51am EST Reply - Quote - Report Abuse
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>> Re: Converting RAID 0 to RAID 5: Is it possible?
Dr. řaz said:
To answer your question, the only way I can think of that may be possible is imaging. For hardware RAID (doesn't matter which type), the OS should see the entire array as a single logical disk. If you make a file-based image (not block-based) of the logical disk, you should be able to restore it even under a different type of RAID setup. I can't say for sure though, until you try it.

Your own thread from month ago, which I happened to also reply. If image works from RAID to non-RAID, the same should apply from one RAID to another. http://www.hardwareanalysis.com/content/topic/78231/

With 3 identical disks, RAID 5 waste 1 disk for parity, leaving you with 2 disks of space. RAID 0 on 2 disks, gives you 2 disks of space. So you have no size mismatch issue here, either, as you already mentioned.

One thing I do want to question is why do you choose RAID 5? RAID 5 is stripe with parity, which distributes stripe blocks and parity data across all drives in the array. The AMD SB950 controller you have doesn't give you much utility for repairing the array in case of a drive failure. Each disk in a RAID 5 is pretty much useless without the controller. In another word, you can't get any useful data out of it by moving it to another PC. RAID 1 is the only RAID that will give you full disk clone, which you can take out and read from another PC, which is more useful. RAID 5 is pretty overkill for a gaming machine really. It doesn't give you much performance gain, since you're already using SSD's, and the controller uses large chunk of resource from Windows since it's not a full hardware RAID! Doesn't give you much data redundancy either, since the onboard controller is limited in functionality.


Over the last two years, the SSD hosting my operating system has failed three times: twice with OCZ and now once with SanDisk. (The second time, I had two SSDs in a RAID 0 hosting the OS so I was dancing with fate anyway.) So 75% of every SSD I have owned has failed within a year of purchase. People have been saying for years how reliable SSDs are as compared to HDDs but I have never had nor have I ever personally seen an HDD fail.

I am absolutely sick of reinstalling Windows and losing all of my settings and many of my programs. (In addition to the drive failures, I've had to reinstall Windows three more times in the last two years.) I want to try and prevent this sort of thing from happening again so it was either switch to a HDD (too slow) or go for RAID. The reason I chose RAID 5 over RAID 1 is because it is an ideal compromise between additional storage and redundancy. I get two drives worth of storage and guaranteed protection against one failure for the price of three drives. I'm not worried about the negligible performance loss. I'm going to go for the pessimistic (and, as far as I see it, perfectly reasonable) assumption that one of these drives is guaranteed to fail within a year.

I don't know how easy it is to rebuild a RAID 5 array after a drive fails but I can only imagine it's not impossible as I think you implied earlier. What would be the point in offering RAID 5 if it can't work after a drive failure? I also don't intend to move the drives to a new computer so the lack of compatibility with other controllers is not an issue unless my motherboard dies, in which case I am also keeping system-image backups.

Normally the price of two new SSDs would bother me but Micro Center had a great sale on a couple 120 GB ones.

The silver lining to this ordeal was that I had an excuse to install Windows 8 which, despite my hatred of the start screen, is an otherwise amazing OS.

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"The more we sweat in peace, the less we bleed in war."
-Patton
Dr. Peaceful Nov 06, 2012, 08:24am EST Reply - Quote - Report Abuse
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Edited: Nov 06, 2012, 08:25am EST

 
>> Re: Converting RAID 0 to RAID 5: Is it possible?
Perhaps my wordings wasn't quite clear in my previous post, below I am making it clearer with additional points. Note that this just for discussion sake in this forum, by all means experiment with whatever you like and see fit.

1. Why is it not a good idea to do complex RAID setup with on-board controllers?
Most of the RAID controllers in motherboards aim for gaming, are only reliable for simple RAID levels such as RAID 0 and RAID 1. Even if the manufacturer claimed their controller is able to do more complex RAID (such as RAID 5, RAID 0+1) for marketing purpose, they probably did not do enough R&D on those due to cost cutting and expectations that the consumers likely won't use those features. Most of these controllers have rather simple ROM Bios for RAID setup, as well as diminutive control programs in the OS. In addition, they may not be full hardware implementations, but depended rather largely on software drivers running in the OS. Furthermore, in case of a motherboard failure, you will likely have a very tough time (or may be impossible) to recover a complicated RAID setup. At minimum, it will require you to replace with a board with the exact same controller. There are good reasons why for enterprise RAID applications, they use dedicated RAID controller boards.

2. Why is it not a good idea to do RAID with SSDs?
The more you write to the SSD, the faster the SSD will wear out. That's one of the inherent drawbacks of SSD. You can see that already from your personal experience with 3 SSD failures in 2 years. RAID may even reduce the life of a SSD faster, due to: one, when RAID first initializes it may need to write to all blocks of the drive; two, it may involve additional writes to the drive, such as writing the parity data. Some RAID setups do not provide the TRIM function, which may reduce the performance of SSD as well. Furthermore, for RAID levels that write across all drives equally, such as RAID 5, wear out all drives at the same rate and increase the chance of multiple drives failure. That makes the "redundancy" in RAID irrelevant.

This has been discussed quite a few times in this forum in the past, such as this thread http://www.hardwareanalysis.com/content/topic/75685/#573861

If you want a more textbook answer, here's a research paper from M$ explained just that.
http://research.microsoft.com/pubs/119126/euro093-balakrishnan.pdf
Right in the beginning of the Abstract:
"SSDs exhibit very different failure characteristics compared to hard drives. In particular, the Bit Error Rate (BER) of an SSD climbs as it receives more writes. As a result, RAID arrays composed from SSDs are subject to correlated failures. By balancing writes evenly across the array, RAID schemes can wear out devices at similar times. When a device in the array fails towards the end of its lifetime, the high BER of the remaining devices can result in data loss."

In page 3, section 2.2:
"The key insight in this paper is that this load-balancing of writes can cause correlated failures in SSD arrays. Since all devices in the RAID-5 array receive similar write workloads, they use up their erasure cycles at similar rates. As a result, the array can be in a state where multiple devices have reached the end of their erasure limit and exhibit high UBERs, making correlated failures highly probable.
...
Importantly, this phenomenon is not specific to RAID-5. For example, it occurs in a fixed-parity configuration (RAID-4) as well, since all data devices age at the same rate. It occurs to a lesser extent in mirrored configurations (RAID-1, RAID-10), since both mirrors age at the same rate. Having two parity devices (RAID-6) prevents data loss on one device failure; however, data loss is likely if two devices are lost. Essentially, any RAID solution used with SSDs offers less reliability than system administrators would like to believe."



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