RAID6 (Redundant Array of Independent Disks level 6) requires a minimum of 4 physical disks to implement properly. This is because RAID6 uses double distributed parity, which means it can withstand the failure of any two disks simultaneously.

The formula for minimum disks in RAID6 is:

N(min) = D + P
Where:
D = Data disks (minimum 2)
P = Parity disks (always 2 for RAID6)
Thus: 2 + 2 = 4

Here's how to check RAID6 configuration in Linux using mdadm:

# Create RAID6 array with 4 disks
sudo mdadm --create /dev/md0 --level=6 --raid-devices=4 /dev/sd{b,c,d,e}

# Check array status
cat /proc/mdstat

With 4 disks in RAID6:

• 2 disks worth of capacity is used for parity
• Only 50% of total storage is available for data
• This overhead decreases as you add more disks

When implementing RAID6:

• Use disks of identical size and performance
• Consider using hardware RAID controllers for better performance
• Monitor disk health regularly
• Have hot spares available for critical systems

RAID6 write operations are slower than RAID5 due to dual parity calculations. Here's a simple benchmark comparison:

# RAID5 write speed (example)
raid5_write: 280 MB/s

# RAID6 write speed (same hardware)
raid6_write: 190 MB/s

RAID6 is a storage configuration that provides dual-disk fault tolerance through the use of two independent parity schemes. Unlike RAID5 which only uses a single parity block, RAID6 can withstand the simultaneous failure of any two drives in the array.

The minimal disk requirement for RAID6 follows this principle:
Minimum disks = Data disks + 2 parity disks
Since we need at least 1 data disk to make the array useful, the absolute minimum becomes:
1 (data) + 2 (parity) = 3 disks

// Example Linux mdadm command to create RAID6 array
sudo mdadm --create /dev/md0 --level=6 --raid-devices=3 /dev/sda1 /dev/sdb1 /dev/sdc1

While 3 disks is theoretically possible, most real-world implementations use 4+ disks for better storage efficiency. The usable storage capacity formula is:
Capacity = (n - 2) * min(disk_sizes)
Where n is the total number of disks.

RAID6 write operations require calculating and writing two parity blocks, which impacts performance. Reading is generally comparable to RAID5. Here's a performance benchmark comparison:

# Sample fio benchmark for RAID6
[global]
ioengine=libaio
direct=1
runtime=60
size=1G

[write-test]
rw=randwrite
bs=4k
numjobs=4

For optimal performance, consider hardware RAID controllers with XOR acceleration or modern filesystems like ZFS that implement RAIDZ2 (similar to RAID6).

When two disks fail in a properly configured RAID6 array, replacement and rebuild is straightforward:

# Replace failed disks (example)
sudo mdadm /dev/md0 --remove /dev/sda1
sudo mdadm /dev/md0 --add /dev/sdd1
# Array will automatically rebuild