Storage 2: RAID Learning Objectives – To understand the technology drivers leading to RAID arrays – To understand the principles of common RAID configurations – To understand performance and reliability consequences of common RAID configurations – To understand the limitations of RAID failure mode operation and recovery

Technology Trends 1980: Hard disk state of art: 1Gbytes in 14” removable drive 1980’s – most PCs used floppy disks 1990’s – most PCs used hard disks over 1 decade, hard disk sales volumes increased x100 First effect: price reduction Second effect: PCs became drivers of disk technology

(approx) Two distinct markets for disk drives: – Mainframe/minicomputer/servers – (14 inch, high capacity, expensive) – PC – (8 inch to 5.25 inch to 3.5 inch; lower capacity, cheap(er)) “How do we build server-class storage using PC-class components?”

RAID in c.1990 PC disks are small => use lots of them PC disks are unreliable – how can we use lots to increase reliability Clue is in the “R” – “Redundant” Use “Redundancy” to provide some error immunity We’ve already seen this in “Disk Mirroring” – see Storage 1 What else can we do?

A Diversion into Marketing 1985: Redundant Array of Inexpensive Disks

A Diversion into Marketing 2010: Redundant Array of Inexpensive XXXXXX Individual Disks Don’t expect “inexpensive”!!!

RAID 0 Striping – (as per previous lecture) All images © Colin M.L. Burnett

RAID 1 Mirroring – (as in previous lecture)

RAID 2 (Bitwise) Hamming Code – not widely used

RAID 3 Striping of sub-blocks, with parity Can operate correctly with signalled errors

RAID 4 Striping of blocks, with parity Performance issue: 1-block write needs R-M-W

RAID 5 Parity is distributed across multiple drives Distributes R-M-W

RAID 6 Multiple Redundancy

On Failure – An Entire Disk (RAID 2-6) Operate in degraded mode (every OS read needs to read every disk) Replace drive (hot-swap?) Rebuild array – how long? – Sequentially – On live system? Failure during rebuild?

Where to Implement RAID? a)in OS b)in Device Interface (RAID controller) – OS-independent – but how to provide User Interface?

For Next Time Does RAID implemented on disk controller suggest ways of building better systems? For the larger scale systems…