But generally, RAID1 gets you 50% of storage space, whereas RAID5 gets you 66% (and any odd combination of disks).
But… you have to weigh it against losing all your data. With raid5 if any 2 of your disks die at the same time then you've lost your volume and most likely 100% of your data (no matter how many disks you have).
With raid1 you have to lose _all_ your disks before that happens. Typically that's also just 2 but you can mirror 3 or more drives if you need some data to be _really_ resistant to disk failures and you don't care about "wasting" n-1 times the space.
So you end up trading off efficiency of storage space and resiliency to data loss.
Myself, disks got cheap enough that I always just buy 2 disks and mirror them. I find it easier to reason about overall, especially in the face of a degraded array.
Why does everyone insist on RAID5? I am completely fine with btrfs RAID1, survived few disk crashes as advertised.
- it will not mount a RAID in degraded mode automatically
it will if using -o degraded, it's the tooling which generally sucks and won't support this. I had problems setting up / on multi-device btrfs using recommended tools (dracut, grub-mkconfig), the bugs are known and unfixed, ended up rolling out own initramfs.
- swapfile support exists, but it breaks snapshots
you are supposed to have all your stuff in a subvolume, and snapshot that, not whole toplevel root filesystem...and yes it should be documented better that it interferes with snapshots
- but snapshots are not integrated..
yep tooling sucks