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Snapraid help

407 Views 8 Replies 3 Participants Last post by htpcforever, Mar 29, 2015

ikkuranus

Discussion Starter · Mar 27, 2015

I decided that snapraid would be the best software raid to fit my needs. I read on the official site that if you use several drives (including the parity drive) of identical size that you wouldn't be able to fill all of the data drives to the brim.

If I were to span the parity drive with a small ssd would it negate this problem? I fully understand that it would add one more point of failure. The reason I am considering this is I will be upgrading to a much bigger ssd which would leave me with several smaller drives that have more than 90% writes available.

I plan to do all of this with 3x4TB Hitachi Deskstar NAS + Crucial M4 128GB or Corsair Force GT 60GB. This will likely be expanded to 5x4TB drives by the end of the year.
My other choice would be to buy one 5TB version of that drive, which is considerably more expensive.

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smitbret

· #2 · Mar 27, 2015

ikkuranus said:
I decided that snapraid would be the best software raid to fit my needs. I read on the official site that if you use several drives (including the parity drive) of identical size that you wouldn't be able to fill all of the data drives to the brim.

If I were to span the parity drive with a small ssd would it negate this problem? I fully understand that it would add one more point of failure. The reason I am considering this is I will be upgrading to a much bigger ssd which would leave me with several smaller drives that have more than 90% writes available.

I plan to do all of this with 3x4TB Hitachi Deskstar NAS + Crucial M4 128GB or Corsair Force GT 60GB. This will likely be expanded to 5x4TB drives by the end of the year.
My other choice would be to buy one 5TB version of that drive, which is considerably more expensive.

Sounds like your OS will be on the SSD, right? SnapRAID won't affect or use that at all. With 3x4TB, you will have roughly 8TB of storage since the 3rd HDD will be used for parity. There's no reason you shouldn't be able to "fill them to the brim".

I think you are confusing what happens when you mix drive sizes. You will always be limited by the size of the Parity Drive. If you have a 2TB Parity Drive and a 3TB Data Drive, you will only be able to use 2TB on the Data Drive. As long as the Parity Drive is as big as the largest Data Drive you can use all of the space.

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htpcforever

· #3 · Mar 27, 2015

Also, it is never a good idea to actually fill them to the brim. Always leave yourself a dozen megabytes free at least.

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smitbret

· #4 · Mar 28, 2015

I don't know what SnapRAID does, but FlexRAID will leave about 50MB open and then start writing to the next drive in the array.

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ikkuranus

Discussion Starter · #5 · Mar 28, 2015

htpcforever said:
Also, it is never a good idea to actually fill them to the brim. Always leave yourself a dozen megabytes free at least.

I was under the impression it would be more on the lines of 15+ GB per data drive due to parity overhead. If it's only in megabytes I could live with that.

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smitbret

· #6 · Mar 28, 2015

ikkuranus said:
I was under the impression it would be more on the lines of 15+ GB per data drive due to parity overhead. If it's only in megabytes I could live with that.

After a cursory glance at the forums, I see a couple of mentions of some parity overhead that is stored on the parity drive but nothing that implies there is any parity on the data drives.

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ikkuranus

Discussion Starter · #7 · Mar 28, 2015

smitbret said:
After a cursory glance at the forums, I see a couple of mentions of some parity overhead that is stored on the parity drive but nothing that implies there is any parity on the data drives.

Correct, but it is proportional to how much data is on the data drives. This is why I asked if a smaller drive spanned with the parity drive would be enough to offset that extra overhead.

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smitbret

· #8 · Mar 28, 2015

ikkuranus said:
Correct, but it is proportional to how much data is on the data drives. This is why I asked if a smaller drive spanned with the parity drive would be enough to offset that extra overhead.

Got it.

Personally, I don't see the use in spanning an SSD with the parity drive. I doubt that the overhead would be noticeable and for less than the cost of the add'l SSD you could have just purchased a larger parity drive and made it a complete non-issue.

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htpcforever

· #9 · Mar 29, 2015

ikkuranus said:
I was under the impression it would be more on the lines of 15+ GB per data drive due to parity overhead. If it's only in megabytes I could live with that.

I was a bit off in my suggestion.

From the forums, it is implied you should not fill any drive more than 99% full. So for a 4TB drive, which is 4000GBs, which means you should keep 40GB free, or roughly the space needed for one large movie. Basically, you are sacrificing a scant 1% of the drive for stability.

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