QNAP VS-2004L en - Manual (Page 173)

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RAID 5 Disk Volume
The data are striped across all the drives in
a RAID 5 array. The parity information is
distributed and stored across each drive.
If a member drive fails, the array enters
degraded mode. After installing a new
drive to replace the failed one, the data
can be rebuilt from other member drives
that contain the parity information.
To create a RAID 5 disk volume, a
minimum of 3 hard disks are required.
The storage capacity of a RAID 5 array
equals (N-1) * (size of smallest hard
drive). N is the total number of hard
drive members in the array.
RAID 6 Disk Volume
The data are striped across all the drives in
a RAID 6 array. RAID 6 differs from RAID
5 that a second set of parity information is
stored across the member drives in the
array. It tolerates failure of two member
drives.
To create a RAID 6 disk volume, a
minimum of 4 hard disks are required.
The storage capacity of a RAID 6 array
equals (N-2) * (size of smallest hard
drive). N is the total number of hard
drive members in the array.