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I'd like to configure a pair of 4TB SSD drives in RAID1.

I consider that when in several years one of the drives fails, and I would want to exchange it, the exact model I have now might be unavailable.

I have impression that "4TB" isn't exactly the same size in various models from various manufacturers. I guess that if the replacement drive is even just slightly smaller than the one I use now, I would be unable to recreate the RAID array?

When configuring RAID1, should I leave some little not allocated space on the drives, to make sure I can use another SSD model as a replacement in the future? How much space should I leave? By how much the real capacity of the SSD drives may vary between various producers and models?

I consider that since it is not recommended to fill SSDs with data to 100% anyway, even if I leave 5% space not allocated to RAID, it should have no practical impact on the operation. Is my logic correct?

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I'm trying to make a RAID 0 of two Samsung 860 EVO 2TB, one is manufactured in 2018 and the other one in 2019 at first the raid failed because there wasn't "enough free space" there is a less than 1MB difference in space on the drives even though it is the same exact model (one had 01 firmware and the other one 03, I updated both drives to the latest 04 firmware before trying to create this RAID) the array was created successfully when I choose the "smaller" drive first. I don't know how common this is, it has never happened to me before, I have just "decommissioned" a 8x1TB RAID 0 where all SSD had the exact same size. (yes I know I'm stupid doing RAID 0 with so many drives, but this is on my desktop with a minimum of a 3-2-1 backup of important data)

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Is it a good practice not to use all the disk space for RAID?

No.

You're right about the potential for smaller drives but you'll be able to see exactly what you've bought and buy the same or bigger then. Also consider buying one or more now if you're that bothered.

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  • Buying an extra SSD now is an option, but that's expensive. And not doing this limits options to buy a replacement in a (possibly far) future. Is there an actual downside in leaving some small space not allocated? I would assume the drive to distribute the wear evenly through the whole space, and I shouldn't be trying to use full capacity anyway.
    – user1079505
    Jul 20, 2022 at 22:41
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    No downside other than lost capacity.
    – Chopper3
    Jul 21, 2022 at 9:40
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Linux md-raid and ZFS (the latter for other, historical reasons) already reserve some MB for pairing slightly mismatched disks - ie: a difference of some sectors can be accommodated without further hassle. Anyway, nowadays disks of the same size have the same number of sectors, so the point is somewhat moot.

That said, reserving some space can be useful for improving performance and endurance if/when using consumer grade (or otherwise low-end) SSDs: in this case, the added unallocated space space helps avoiding frequent rewrites and slowdowns due to the controller's garbage collector activity. For enterprise-grade SSDs this is not generally required, so I would use the entire disk space.

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