ES-1 Elettronica dei Sistemi 1 Computer Architecture - diegm

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					    ES-1 Elettronica dei Sistemi 1
       Computer Architecture


                        Lesson 7
                        Disk Arrays




1              es-1-7
      Network Attached Storage

        14" » 10" » 8" » 5.25" » 3.5" » 2.5" » 1.8" » 1.3" » . . .
        high bandwidth disk systems based on arrays of disks
                                                  Decreasing Disk Diameters


    Network provides           High Performance
                               High Performance       Network File Services
    well defined physical       Storage Service
                                 Storage Service
    and logical interfaces:     on a High Speed
                                 on a High Speed
    separate CPU and                Network          OS structures
                                     Network         supporting remote
    storage system!
                                                     file access


           3 Mb/s » 10Mb/s » 50 Mb/s » 100 Mb/s » 1 Gb/s » 10 Gb/s
           networks capable of sustaining high bandwidth transfers

       Increasing Network Bandwidth
2                                                                        es-1-7
                    Manufacturing Advantages
                    of Disk Arrays



    Conventional:
    4 disk
    designs   3.5” 5.25”    10”    14”

               Low End            High End

    Disk Array:
    1 disk design
       3.5”
3                                            es-1-7
                      Replace Small # of Large Disks
     with Large # of Small Disks! (1988 Disks)



                    IBM 3390 (K)   IBM 3.5" 0061      x70
    Data Capacity    20 GBytes     320 MBytes      23 GBytes
    Volume           97 cu. ft.      0.1 cu. ft.   11 cu. ft.
    Power              3 KW            11 W          1 KW
    Data Rate         15 MB/s        1.5 MB/s      120 MB/s
    I/O Rate         600 I/Os/s      55 I/Os/s     3900 IOs/s
    MTTF             250 KHrs        50 KHrs        ??? Hrs
    Cost               $250K            $2K          $150K



4                                                               es-1-7
    Array Reliability


    • Reliability of N disks = Reliability of 1 Disk ÷ N
          50,000 Hours ÷ 70 disks = 700 hours

          Disk system MTTF: Drops from 6 years to 1 month!

    • Arrays (without redundancy) too unreliable to be
    useful!


      Hot spares support reconstruction in parallel
      Hot spares support reconstruction in parallel
      with access: very high media availability can be
      with access: very high media availability can be
      achieved
      achieved
5                                                            es-1-7
    RAID

    ?   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
6                                                      es-1-7
                          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)

                                Horizontal Hamming Codes (overkill)
         Techniques:
                                Parity & Reed-Solomon Codes
7                               Failure Prediction (no capacity overhead!)
                                                                       es-1-7
    RAID 0
                                                              ?
    ? 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
                             R        O       U      D
                             R        O       B      I
                             N
8                                                               es-1-7
                  Redundant Arrays of Disks
    RAID 1: 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: 100% capacity overhead

9     Targeted for high I/O rate , high availability environments   es-1-7
     RAID 1: 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

10                                                 es-1-7
     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
11                                                 es-1-7
     Hamming Code

      K4 K3 K2 K1   C1 = D1?   D2 ?   D4 ?   D5 ? D7
       1 1 0 0 D8   C2 = D1?   D3 ?   D4 ?   D6 ? D7
       1 0 1 1 D7   C3 = D2?   D3 ?   D4 ?   D8
       1 0 1 0 D6   C4 = D5?   D6 ?   D7 ?   D8
       1 0 0 1 D5
       1 0 0 0 C4   Esempio:
       0 1 1 1 D4   Dato: 0 1 0 0 1 1 0 0
       0 1 1 0 D3   D-err: 0 0 0 0 1 1 0 0
       0 1 0 1 D2
       0 1 0 0 C3   C4= 1, C3= 0 C2= 1 C1= 0
       0 0 1 1 D1   ?
       0 0 1 0 C2   C4= 0, C3= 0 C2= 0 C1= 1
       0 0 0 1 C1     1      0     1     1
12                                                es-1-7
     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



13                                                  es-1-7
        RAID-3: Parity Disk

          10010011
          11001101                                                    P
          10010011
             ...
        logical record                  1         1          1         0
                                        0         1          0         0
        Striped physical                0         0          0         1
             records                    1         0          1         1
                                        0         1          0         0
                                        0         1          0         0
                                        1         0          1         0
                                        1         1          1         0


     • Parity computed across recovery group to protect against
       hard disk failures



14   Targeted for high bandwidth applications: Scientific, Image Processing
                                                                           es-1-7
     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




15                                                      es-1-7
     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


     ?   N.B. DOES NOT MEAN 5 DISKS!!!!!


16                                                es-1-7
     RAID 5+: High I/O Rate Parity
                                                            Increasing
                                                              Logical
     A logical write
      A logical write       D0    D1     D2     D3    P        Disk
     becomes four
      becomes four                                          Addresses
     physical I/Os
      physical I/Os
                            D4    D5     D6     P     D7
     Independent writes
      Independent writes
     possible because of
      possible because of
     interleaved parity
      interleaved parity    D8    D9     P     D10    D11
     Reed-Solomon
      Reed-Solomon
     Codes ("Q") for
      Codes ("Q") for       D12   P     D13    D14    D15     Stripe
     protection during
      protection during
     reconstruction
      reconstruction
                            P     D16   D17                   Stripe
                                               D18    D19      Unit

                            D20   D21   D22     D23    P
                             .     .      .      .     .
     Targeted for mixed      .     . Disk .Columns     .
                                                 .
17   applications            .     .      .      .     .       es-1-7
                          Problems of RAID-5 Arrays:
                          Small Writes
     RAID-5: Small Write Algorithm
        1 Logical Write = 2 Physical Reads + 2 Physical Writes

        D0'          D0      D1        D2    D3       P


     new              old                            old
     data             data (1. Read)                 parity (2. Read)
                 + XOR


                                     + XOR



                     (3. Write)                    (4. Write)


18                 D0'      D1       D2      D3      P'
                                                                        es-1-7
       Subsystem Organization
                                                 single board
                    host       array                 disk
     host
                   adapter   controller           controller

     manages interface                           single board
     to host, DMA                                    disk
                                                  controller
     control, buffering,
     parity logic
                                                 single board
                                                     disk
     physical device                              controller
     control

      striping software off-loaded from          single board
            host to array controller                 disk
                                                  controller
        no applications modifications
                                          often piggy-backed
      no reduction of host performance    in small format devices
19                                                                  es-1-7
                                          System Availability:
                                          Orthogonal RAIDs
                     String
                    Controller                           . . .

                     String
                    Controller                           . . .

                     String
       Array        Controller                            . . .
     Controller
                     String
                    Controller                            . . .

                     String
                    Controller                            . . .

                     String
                    Controller                            . . .



     Data Recovery Group: unit of data redundancy
     Redundant Support Components: fans, power supplies, controller, cables
20   End to End Data Integrity: internal parity protected data paths   es-1-7
                             System-Level Availability


      host                                           host
     I/O Controller     Fully dual redundant I/O Controller



     Array Controller                         Array Controller
      ...                                                ...

                                        ...
                                                          Goal: No Single
                                                          Goal: No Single
                                                            Points of
                                                             Points of
                                        ...                  Failure
                                                               Failure


                                        ...

                  .       with duplicated paths, higher performance can be
      Recovery    .              obtained when there are no failures
21    Group       .                                                         es-1-7

				
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