My current NAS is a bunch of old hardware slapped together, I’m thinking of downsizing it to a proper NAS box. I was curious if anyone had any advice on whether a 2 bay with two 18tb drives mirrored, or a 4 bay with 4 6tb drives in raid 5 would make more sense. Same capacity, same amount of allowed drive failures. So what are the pros and cons to each setup?
I’d suggest a different option, if budget allows:
- 4x 16TB in RAID6 gives you 32TB storage and 2 (!!) redundant drives
- 3x 20TB in RAID5 gives you 40TB storage and 1 redundant drive
Hunt around for deals on used enterprise drives, but make sure you limit the amount of drives from a single manufacturer to the amount of redundant drives you choose to have. So for RAID5, each drive comes from a different brand, whereas on RAID6 you can have 2 sets of 2 drives each per manufacturer.
HTH!
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Apart from redundancy calculation I’d like to throw these two in here:
- More spindles often means better IO, but obviously "it depends"™
- Less spindles mean less power draw, idle and on access
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I think meilon get it all in the software side.
With 4 drive you could have a raid Z2 if you don’t mind looking space.
Between 6 and 18 To you may get a small performance difference, principaly in sequential.
And you have the space/heat/noize related to having more drive.
The cost per TB for new drives is better at 16 or 18 To.
The worst-case failure is always going to be that one day, one drive just doesn’t spin up. I’ve had this happen more than once. So, the question is ‘how big of a PITA is it for you to replace a whole drive’. I can’t answer that question for you, only you can answer that question.
A normal hard drive is going to be able to do about 150MB/s when it’s empty, and closer to 75MB/s when it’s nearly full. We’ll average it out and call it 100MB/s assuming 0 random access and large files, like movies. 18,000,000 / 100 = 180,000 seconds to fill the drive. That’s 50 hours. So it’s going to take you at least 2.5 days to recover from a drive loss, because you’re not going to get perfect transfer rates. If we are talking about rebuilding from parity data, that number might be closer to 3-5 days.
Because your drives are a hodgepodge, the odds that you lose 2 drives back to back is actually lower than if you bought all the drives together at the same time. The most important thing with any NAS is to stay on top of drive failures. Don’t be like every tech youtuber who has dead drives in the array. “Cleanliness is next to Godliness” applies here. Keep your NAS healthy, and your life will be far less stressful.
In both cases (mirror/raid5) the write speed is limited by the slowest drive. Until you get over 700TB tape storage won’t make sense as the other part of the ‘mirror’ (check numbers for your country if not in the US). Start with LTO-5 and work your way up. Cost of drive + cost of tapes to store data. Note: Tapes need to be stored in a cool, dry, place. Don’t forget HBA costs.
Now, on to pro/con:
Mirroring:
- Pro - If one drive breaks, the remaining drive has a complete copy of all data in a useable format. You lose redundancy, not functionality.
- Read speed varies by implementation. Sometimes it is the sum of both drives, sometimes you are limited to whichever drive is assigned that read. It depends on how the RAID is implemented.
- Con - More expensive. Double the $/TB for whatever drive(s) you buy to get your true cost of storage.
Raid 5:
- Pro - Cheaper. You get more usable space vs mirroring.
- Pro - Read speed is the sum of all drives. So 4 drives means 400MB/s read speed under ideal conditions (files large enough to be stripped across all 4 drives, sequential access).
- Con - If one drive fails read speed is negatively impacted as data on that drive must be reconstructed from parity on the fly. If more than one drive fails, data is an un-usable mess. Not even single files can easily be reclaimed from what is left of the array as they are scattered in stripes across all disks.
- Con - More disks means you need HBAs, chassis, or other ‘extra’ components sooner. For 2 vs 4 drives, not a big deal. But as you expand, it can make a difference.
- 2x the power consumption of mirroring. Drives use, more or less, the same power whether they are 6TB of 24TB. The power draw isn’t crazy, but 20W per drive adds up.