ES-1 Elettronica dei Sistemi 1 Computer Architecture

Transcription

1 ES- Elettronica dei Sistemi Computer Architecture Lesson 7 Disk Arrays

2 Network Attached Storage 4"» "» 8"» 525"» 35"» 25"» 8"» 3"» high bandwidth disk systems based on arrays of disks Decreasing Disk Diameters Network provides well defined physical and logical interfaces: separate CPU and storage system! High High Performance Storage Service on on a High High Speed Speed Network Network File Services OS structures supporting remote file access 3 Mb/s» Mb/s» 5 Mb/s» Mb/s» Gb/s» Gb/s networks capable of sustaining high bandwidth transfers 2 Increasing Network Bandwidth

3 Manufacturing Advantages of Disk Arrays Conventional: 4 disk designs Low End High End 3 Disk Array: disk design 35

4 Replace Small # of Large Disks with Large # of Small Disks! (988 Disks) IBM 339 (K) IBM 35" 6 x7 Data Capacity 2 GBytes 32 MBytes 23 GBytes Volume 97 cu ft cu ft cu ft Power 3 KW W KW Data Rate 5 MB/s 5 MB/s 2 MB/s I/O Rate 6 I/Os/s 55 I/Os/s 39 IOs/s MTTF 25 KHrs 5 KHrs??? Hrs Cost $25K $2K $5K 4

5 Array Reliability Reliability of N disks = Reliability of Disk N 5, Hours 7 disks = 7 hours Disk system MTTF: Drops from 6 years to month! Arrays (without redundancy) too unreliable to be useful! 5 Hot spares support reconstruction in in parallel with access: very high media availability can be be achieved

6 RAID 6 Redundant Array of Independent Disks Redundant Array of Inexpensive Disks 6 levels in common use Not a hierarchy Set of physical disks viewed as single logical drive by O/S Data distributed across physical drives Can use redundant capacity to store parity information

7 Redundant Arrays of Inexpensive Disks Files are "striped" across multiple spindles Redundancy yields high data availability Disks will fail Contents reconstructed from data redundantly stored in the array Capacity penalty to store it Bandwidth penalty to update Mirroring/Shadowing (high capacity cost) 7 Techniques: Horizontal Hamming Codes (overkill) Parity & Reed-Solomon Codes Failure Prediction (no capacity overhead!)

8 RAID No redundancy Data striped across all disks Round Robin striping Increase speed Multiple data requests probably not on same disk Disks seek in parallel A set of data is likely to be striped across multiple disks 8 R O U D R O B I N

9 Redundant Arrays of Disks RAID : Disk Mirroring/Shadowing recovery group Each disk is fully duplicated onto its "shadow" Very high availability can be achieved Bandwidth sacrifice on write: Logical write = two physical writes Reads may be optimized Most expensive solution: % capacity overhead 9 Targeted for high I/O rate, high availability environments

10 RAID : summary Mirrored Disks Data is striped across disks 2 copies of each stripe on separate disks Read from either Write to both Recovery is simple Swap faulty disk & re-mirror No down time Expensive

11 RAID 2 Disks are synchronized Very small stripes Often single byte/word Error correction calculated across corresponding bits on disks Multiple parity disks store Hamming code error correction in corresponding positions Lots of redundancy Expensive Not used

12 Hamming Code 2 K4 K3 K2 K C = D? D2? D4? D5? D7 D8 C2 = D? D3? D4? D6? D7 D7 C3 = D2? D3? D4? D8 D6 C4 = D5? D6? D7? D8 D5 C4 Esempio: D4 Dato: D3 D-err: D2 C3 C4=, C3= C2= C= D? C2 C4=, C3= C2= C= C

13 RAID 3 Similar to RAID 2 Only one redundant disk, no matter how large the array Simple parity bit for each set of corresponding bits Data on failed drive can be reconstructed from surviving data and parity info Very high transfer rates 3

14 RAID-3: Parity Disk P logical record Striped physical records Parity computed across recovery group to protect against hard disk failures 4 Targeted for high bandwidth applications: Scientific, Image Processing

15 RAID 4 Each disk operates independently Good for high I/O request rate Large stripes Bit by bit parity calculated across stripes on each disk Parity stored on parity disk 5

16 RAID 5 Like RAID 4 Parity striped across all disks Round robin allocation for parity stripe Avoids RAID 4 bottleneck at parity disk Commonly used in network servers NB DOES NOT MEAN 5 DISKS!!!!! 6

17 RAID 5+: High I/O Rate Parity A logical logical write write becomes four four physical I/Os I/Os Independent writes writes possible because of of interleaved parity parity D D D2 D3 P D4 D5 D6 P D7 D8 D9 P D D Increasing Logical Disk Addresses Reed-Solomon Codes Codes ("Q") ("Q") for for protection during during reconstruction D2 P D3 D4 D5 P D6 D7 D8 D9 Stripe Stripe Unit D2 D2 D22 D23 P 7 Targeted for mixed applications Disk Columns

18 RAID-5: Small Write Algorithm Problems of RAID-5 Arrays: Small Writes Logical Write = 2 Physical Reads + 2 Physical Writes D' D D D2 D3 P new data old data ( Read) old parity (2 Read) + XOR + XOR (3 Write) (4 Write) 8 D' D D2 D3 P'

19 Subsystem Organization host host adapter array controller single board disk controller manages interface to host, DMA control, buffering, parity logic physical device control single board disk controller single board disk controller 9 striping software off-loaded from host to array controller no applications modifications no reduction of host performance single board disk controller often piggy-backed in small format devices

20 Array Controller String Controller String Controller String Controller String Controller String Controller String Controller System Availability: Orthogonal RAIDs 2 Data Recovery Group: unit of data redundancy Redundant Support Components: fans, power supplies, controller, cables End to End Data Integrity: internal parity protected data paths

21 System-Level Availability host I/O Controller Fully dual redundant host I/O Controller Array Controller Array Controller Goal: Goal: No No Single Single Points Points of of Failure Failure 2 Recovery Group with duplicated paths, higher performance can be obtained when there are no failures