Raid

RAID

Redundant Array of Independent Disks

Is not a backup
- If your operating system or software, rather than the hard disk, corrupts your data, this corrupted data is sent to both disks and simultaneously corrupts both drives.

Striping (Disk A and B contain data split between them that together forms the whole)

Combination of RAID 0 and RAID 1
Requires at least 4 drives
- Drives should be identical (The disk geometry (number of heads, cylinders, etc.) is critical and it is strongly recommended NOT to use dissimilar disks.)
Protects you from a single drive failure
- reads the surviving mirror and stores the copy to the new drive you replaced. (Not nearly as taxing of an operation as RAID 5)
cuts your usable disk space in half 4x2TB Disks == 4TB total storage

consists of block-level striping with a dedicated parity disk.
As a result of its layout, RAID 4 provides good performance of random reads
while the performance of random writes is low due to the need to write all parity data to a single disk

When a drive fails
- it needs to read everything on all the remaining drives to rebuild the new, replaced disk (A heavy load for the surviving disk and potential failure point of the 2nd disk)
Storage Volume ((Number of hard drives - 1) x storage capacity of the smallest hard drive)
- 3x1TB drives == 2TB storage, 3rd disk is for parity data
Only at risk for failure if at least 2 drives fail simultaneously
- That’s why, typically, an odd number of data carriers, i.e., three, five, seven, etc., is combined.

Combines four or more hard drives into a single logical drive.
Often referred to as “RAID 5 expansion”
Striping (All data is divided into blocks and distributed evenly to the participating hard disks.)
Parity (always saves two sets of parity information. In that way, associated data can be restored if one or two disks fail.)
Storage Volume ((Number of hard drives - 2) x space of the smallest hard drive)
- For example, with four 1GB hard disks, only 50% of their potential memory would be available to store user data. However, as the number of disks increases, this relationship between capacity and parity improves.
Advantage over RAID 5
- parity information to recover lost data is saved in duplicate. Duplicated parity data is a more efficient way of creating redundancy, and also ensures higher reliability.
- This is less taxing on the remaining drives compared to ordinary