SSD Raid 0 questions

dgingeri

dgingeri

2[H]4U
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Dec 5, 2004
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Anyone out there with experience with RAID 0 and SSDs? Specifically, I'd like to know if anyone has experience with Sandforce SSDs in a RAID 0 format.

I know TRIM isn't transferred to a RAID array, but I'm thinking that won't be an issue. The reason I don't think it will be an issue is that the RAID striping won't generally cross the page lines. it will be writing more than a page at a time per drive, even with the lowest setting. So, if you have a pair of SSDs in a RAID 0 with a 8k stripe, it will be writing 4k per drive at a time, so TRIM and garbage collection won't even be needed. That is assuming the controller lines the stripes up properly. If the controller lines the stripes up wrong, it could seriously degrade performance.

Has anyone set up drives like this, and have you been able to avoid the performance degradation associated with partial page writes? What controller are you using, and how many have you tried? Did you get the performance you were expecting?

I have a feeling this will become more and more common in the near future, and RAID controller manufacturers might actually begin to offer special options for SSD RAIDs.

I have already purchased the drives I intend to use, 3.5" Vertex 2 drives from Newegg, (they had them for $207 each and one $30 MIR) used with a 3Ware 9650SE 8 port controller in the PCIe x4 slot of a Asus P6T. Overclocked 920 to 4.2GHz, 12GB Corsair Vengeance memory, dual GTX470 video cards round out the system.

Currently, I have a 4X 750GB Caviar Black raid 0 setup, but the transfer rate isn't nearly what I thought it should be. I'm only getting 300MB/s out of them. I'm hoping to get at least 500MB/s from these SSDs. I'm going to hook them to my controller and test out various raid striping and test out the drives for a little while before making them my OS drive.
 
Me thinks you don't quite get why TRIM is important:

http://www.makeuseof.com/tag/why-trim-important-to-solid-state-hard-drives/

"Usually, a flash memory cell will have larger memory chunks called “blocks” which are made up of smaller memory chunks called “pages”."

"Whenever you try to write data to a page which is used, you encounter something called the “block-rewrite penalty”. Pages are very specifically arranged in blocks. If you have to replace the data in a page, you have to rewrite all the data on the entire block. A page is usually 4KB, and a block is usually 512KB, so suddenly you’re dealing with far more data. All of the information in the block has to be written into the SSD’s cache, so that it is not lost, then the data is re-written to the entire block and all of its pages."

So if you had a 1024k stripe (512k, or one block per drive), then you wouldn't have a problem and wouldn't need TRIM. But that's a pretty big stripe.. especially for a relatively smaller drive that most SSDs are.

And this assumes the Load Leveling logic of the Firmware doesn't get involved and rearrange pages that cross block lines.. I have no clue how that might or might not work as each SSD firmware is proprietary.
 
oh, it erases the whole block?? I thought it was just a page at a time. Wow, that's a harsh penalty. They really should try to design that better.

Well, I'll be giving it a try. I guess I'll give it 256k stripes, as that is the maximum for my controller. Good thing it has 256MB of cache memory.
 
TRIM is half the story. Garbage Collection is a bit harder to grasp, but it is how newer drives maintain good performance in RAID even without TRIM. Best is TRIM + GC, but GC and no TRIM is better than having neither.
 
dgingeri said:
oh, it erases the whole block?? I thought it was just a page at a time. Wow, that's a harsh penalty. They really should try to design that better.

Well, I'll be giving it a try. I guess I'll give it 256k stripes, as that is the maximum for my controller. Good thing it has 256MB of cache memory.
Click to expand...

I am sure they want to, but to simplify the problem: if you want to only rewrite individual pages or cells (best option) each page, or each cell, would need it's own wiring back to the Flash controller.. your internal wiring in the flash chips start to become more real estate than the actual memory..
 
IMO RAID 0 is not worth it for gaming and general Windows usage. I'm running two Intel SSDs in RAID 0 and best I can tell is 1-2 seconds of load and install time for most apps and games of considerable size. Came from over a year on a single drive.

Day to day I can't tell the difference and wish I had a single larger SSD. Less risk, less CPU overhead, automated maintenance with trim, less expensive, and better resale.
 
dgingeri said:
oh, it erases the whole block?? I thought it was just a page at a time. Wow, that's a harsh penalty. They really should try to design that better.

Well, I'll be giving it a try. I guess I'll give it 256k stripes, as that is the maximum for my controller. Good thing it has 256MB of cache memory.
Click to expand...

How do 256k stripes do any different for the SSD logic than tiny ones? If it isn't large enough for the entire block it accomplishes nothing for the SSD.

Personally I would get one of the largest SSD that you can / want to afford and not mess with RAID.
 
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