RAID

Redundant Array of Independent Disks

• Key idea is redundancy
• Why?

RAID Types

• Hardware - physical hardware controls RAID
  • More expensive
  • array is presented as a single logical disk to OS
  • manage with tools provided by vendor
  • Cannot nest raid levels
  • hope your raid card never dies
  • may have better performance over software RAID for very large arrays

RAID Types

• Software - kernel takes care of RAID functionality
  • OS has to take physical disks and convert to logical
  • can use any regular block device (paritition, disk, files…)
  • supports RAID nesting (create a RAID 0 of RAID 6)
  • tool: mdadm (supports hot spares, hot swap)
  • Why not? If you have to dual boot.

RAID Types

• Fake - Essentially software raid, provided by the BIOS on the mobo.
  • different from vendor to vendor
  • tool: dmraid
  • usually only supports RAIDing an entire disk, not partitions
  • lose mobo, lose data
  • usually only used when dual-booting (win and linux)

RAID Types - Fake raid again

The terms “Fake RAID” or “BIOS RAID” denote systems with no RAID controller, but with a simple BIOS that is able to do basic RAID operations on an array of disks. For the actual runtime RAID implementation, this RAID solution relies on a OS driver. On Windows this will usually be a driver from the RAID vendor.

Typically more useful for Windows as linux software raid works pretty well.

RAID Levels

There are too many RAID levels that exist. But, we will go over the MOST common ones that you will see in the wild (and the ones supported on our machines):

• 0
• 1
• 5
• 10

RAID 0 - Striping

Makes multiple disks look as if they were a single disk.

Advantages:

• You get more disk space
• Speed?

Disadvantages:

• No redundancy
• LVM Is better

RAID 0 - Striping

RAID 1 - Mirroring

Data blocks are duplicated to another physical disk.

Advantage:

• easy redundancy

Dis:

• Two write operations have to occur per block

RAID 1 - Mirroring

RAID 5 - Parity

Blocks are alternately written to different disks. A parity block is also written. Must have at least 3 disks

• How parity is calculated

RAID 5 - Parity

RAID 10

RAID

See table 11-3 here

Managing RAID

Hardware manufacturers can provide utilities to configure hardware raid. Some commands are also available in the kernel:

• mdadm (for software/fake raid)
• dmraid (for software/fake raid)
  • understands more metadata formats
• cat /proc/mdstat

Mdadm how to

See my cheatsheet

This site might also be useful.

Misc

Raid chunk size - The default when creating an array is 512KB (using mdadm)

• If you choose a chunk size of 64K, a 256K file will use 4 chunks. (If this were raid 0 with 4 drives, this would work nicely)
• What if you had a chunks size of 256K? You’d only use 1 chunk, and the benefits of striping would be negligible.