JBOD - Just a Bunch of Disks

RAID 0 - Striping

• Secure against single theft
• Unsecure against hardware failure

RAID 1 - Mirroring

• Safe against HW failure
• COMPLETELY unsafe against theft
  • Use volume encryption! LUKS, Bitlocker, …
• Hot spare disk = idle disk, if needed a rebuild action can start automatically

Parity calculations: XOR

Calculating parity

data1:  0100 1110
data2:  1010 1011
-----------------
parity: 1110 0101

Recovering data1 using parity:

data2:  1010 1011
parity: 1110 0101
-----------------
data1:  0100 1110

RAID 5 - Striping with parity

• At least 3 disks needed
  • One disk lost for capacity
• Safe against HW failure of one disk
  • One disk fails ⇒ RAID 0-like situation
  • If hot spare is configured, rebuild action starts
  • Warning: increased load on other disks, may trigger other failures
• Current processors make RAID 5 as fast as RAID 1 (or better!)

Parity with 4 disks

block1a:    0100 1110
block2a:    1010 1011
---------------------
parity-tmp: 1110 0101
block3a:    1101 0011
---------------------
parity:     0011 0110

Recovering block2a using parity

block3a:    1101 0011
parity:     0011 0110
---------------------
parity-tmp: 1110 0101
block1a:    0100 1110
---------------------
block2a:    1010 1011

RAID 10 - Mirroring + striping

• Is RAID 1 (mirroring) combined with RAID 0 (striping)
• At least 4 disks needed
  • Half capacity usable!
• Very frequently used in enterprise environments

Software vs Hardware RAID

• Difference = where are the calculations done?
  • HW RAID: dedicated RAID chip (controller - onboard or plugin card)
  • SW RAID: CPU
• Directly Attached Storage (DAS): not much use for HW RAID
  • Main CPU is strong enough for added RAID functionality
  • Recovery/forensic analysis is easier with SW RAID
• Network Attached Storage (NAS)/Storage Area Network (SAN): HW RAID
  • Not much choice, often embedded in the device!
  • Dedicated RAID controller with proprietary firmware

The big problem with HW RAID

• Proprietary firmware
  • In case of data loss: recovery/forensic analysis is difficult/impossible
• Good RAID controller duplicates config on each disk for recovery purposes
  • Not a standard! Depends on implementation
• RAID controllers are aging
  • E.g. RAID 10 on Areca RAID controller is not recognized if the disks are connected to an Adaptec RAID controller
  • OR: RAID 10 on a 5 year old Areca RAID is not recognized in recend Areca RAID controller
  • RAID controller has become a single point of failure!

Software RAID

• Can be done by the OS
  • Linux Software RAID (mdadm), Linux Volume Management (LVM)
  • Windows Dynamic Volumes, Storage Spaces
  • 3rd party software (e.g. Starwind)
• Usually easily expandable
• Recoverable
• Backwards compatible
• “Free” (included in OS)
• Independent of interface (IDE, SATA, SCSI, …)
• Fast (with modern CPUs)

Disadvantages of SW RAID

• CPU load
• OS is not easy to install on a RAID volume
• Not all RAID levels are supported
• Hot swap not always supported