Chapter 6 External Memory. Dr. Mohamed H. Al-Meer

Transcription

1 Chapter 6 External Memory Dr. Mohamed H. Al-Meer

2 6.1 Magnetic Disks Types of External Memory Magnetic Disks RAID Removable Optical CD ROM CD Recordable CD-R CD Re writable CD-RW DVD Magnetic Tape 2

3 Introduction Redundant Array of Independent Disks Redundant Array of Inexpensive Disks Set of physical drives viewed by OS as a single drive Data distributed across devices Redundant disk capacity used to store parity data (used for data recovery from failure) This distribution of data ensures parallel access Improves performance? (how) Handles multiple IO services? (how) 3

4 RAID 0 No redundancy (Not True Member) Used in some supercomputers. Where capacity and performance is critical not redundancy. Can serve 2 IO requests if their data is distributed across 2 disks (parallel request service) Data striped across all disks (see Figure 6.10) Round Robin striping is used Logical disk is divided in strips. Strips may be physical block, sector, or others. Strips are mapped in round robin to consecutive array memembers 4

5 RAID 0 Advantage: can load in parallel different strips in parallel 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 5

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8 RAID 1 Mirrored Disks (Duplication) Data is striped across disks 2 copies of each stripe on separate disks Read from either (one image only) Write to both always. In parallel. No Write penalty as no parity is used here. Recovery is simple Swap faulty disk & re-mirror No down time Expensive. Requires twice disk space. Used for system software and critical data and files 8

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10 RAID 2 Disks are synchronized (each disk head is in same position) Very small stripes. Often single byte/word Error correction has special dedicated disks Error calculated across corresponding parity disks (Hamming Code) Multiple parity disks store Hamming code error correction in corresponding positions Number of parity bits = log 2 number of data disks 10

11 RAID 2 On single bit errors,the controller can recognizes error and correct it instantly so read access time is not slowed down RAID 2 is for environment of high occurrence rates of disk errors (gives high reliability) Lots of redundancy Expensive Not used (over killed by other types) 11

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13 RAID 3 Similar to RAID 2 Only one redundant disk, no matter how large the array (for error checking and correction) Simple parity bit for each set of corresponding bits Data on failed drive can be reconstructed from surviving data and parity information only Very high transfer because very small strips of data distributed across drives 13

14 RAID 3 Parity Formula Assume 5 drives, 4 data drives and 1 for parity check X0 X1 X2 X3 X4 (Parity) Then the formula will be: X4(i) = X0(i) XOR X1(i) XOR X2(i) XOR X3(i) If X1(i) fails then add next term to both sides X4(i) XOR X1(i) Then will get X1(i) = X0(i) XOR X2(i) XOR X3(i) XOR X4(i) 14

15 RAID 3 If one disk fails then data still available (call this reduced mode). For missing Data, it is generated on the fly. IO request will involve parallel transfer of data from all data disks Only one IO request can be initiated at once (Why?) Not good for transaction-oriented environment. 15

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17 RAID 4 RAID4 to 6 uses: Independent access technique. Each disk operate independently to satisfy separate IO request Strip size is large (blocks of data) 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 17

18 RAID 4 Write penalty involved: each time a write occurs, the array management software updates not only user data (eve if small) but also corresponding parity bits. But can be reduced by employing new technique by management software. Example X4(i) = X3(i) XOR X2(i) XOR X1(i) XOR X0(i) Assume X1(i) changed to X1 (i) then X4(i) will be X4 (i) X4 (i) = = X3(i) XOR X2(i) XOR X1 (i) XOR X0(i) 18

19 RAID 4 Example Add X1(i) XOR X1(i) to right side so we get: X4 (i) = X3(i) XOR X2(i) XOR X1 (i) XOR X0(i) XOR X1(i) XOR X1(i) X4 (i) = X4(i) XOR X1(i) XOR X1 (i) So to change parity Ex-OR old parity with old X1 and with new X1. Disadvantage Even for a small update, the parity disk will be accessed IO bottleneck at parity disk 19

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21 RAID 5 Like RAID 4 Parity strips striped (distributed) across all disks Round robin allocation for parity stripe Avoids RAID 4 bottleneck at parity disk found in RAID-4 Commonly used in network servers 21

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23 RAID 6 2different parity calculations computed and stored in 2 different blocks on 2 different disks Have N user data disk plus 2 parity disks always 2 parity disks moving around all disks (Round Robin) One for EXOR check bits (P) and other independent data check algorithm (Q) Regenerate data even if 2 user data drives fail Provides extremely high data availability Requires write penalty as each write affects 2 parity blocks. 23

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