Stripes of mirrors
RAID 10, also written RAID 1+0, pairs the drives into mirrors and stripes data across the pairs. Each pair works like RAID 1, and the stripe on top works like RAID 0. It needs at least four drives and an even count.
Usable space is half the raw space, whatever the drive count. Four 2 TB drives give 4 TB from 8 TB raw, and eight give 8 TB from 16 TB.
The order of the layers matters. RAID 0+1 builds them the other way, as a mirror of two stripe sets. One failed drive takes down its whole stripe set, and the other set runs as plain RAID 0 until you rebuild. RAID 10 loses only the one drive.
Which failures it survives
RAID 10 guarantees one failure. After that it depends on which drive goes next. Each pair can lose one drive, but if both drives of a pair die, the array is gone. With four drives, a second failure that hits one of the three remaining drives at random is survivable 66.7% of the time. With six drives it's 80% and with eight it's 85.7%. Spreading the drives over more pairs thins out the risk.
The drive to worry about is the surviving half of the broken pair. During the rebuild it's read from start to finish, and it holds the only copy of that pair's data.
Why it's the write friendly level
There's no parity to calculate. A write goes to both drives of one pair, two writes in all, and reads can come from either drive of a pair. That's why databases and virtual machine hosts run RAID 10. Small random writes cost less than on RAID 5 or RAID 6, which need extra reads and parity writes.
Rebuilds are short and simple too. A failed drive is replaced by copying its partner, so only one drive gets read. On four 8 TB drives that read is 8 TB, and at the modeled hard drive speed it takes at best 14 hours. RAID 5 on the same drives reads 24 TB. At a spec rate of 1 in 10^14 bits, the chance of an unrecoverable read error in the 8 TB mirror read is at most about 47%, against 85% for the 24 TB a RAID 5 rebuild reads. Both are ceilings, not forecasts.
What it costs
RAID 10 spends half your drives on copies. Four 8 TB drives give 16 TB, and the same drives in RAID 5 give 24 TB. For a write-heavy workload that's a fair trade. For a media library it isn't, and RAID 5 or RAID 6 fits better. Either way, mirrors guard against a dead drive and nothing else, so a backup still belongs somewhere else.