CS537 11/16

Brief

• RAID
  • S/W (file system)
  • H/W (hardware) disks, RAIDs, SSD
• File System
  • API
  • Internals

RAID

• Why?
  • Performance
  • Capacity
  • Reliability (Durability)
• Failure model
  • entire drive:
    • Working or (Completely) failed
    • Easily detected (by RAID controller)
• RAID “Levels”
  • Level 0: no redundancy （striping / JBOD)

    • Disk 0	Disk 1	Disk 2
      Block 0	1	2
      3	4	5

    • No redundancy: can’t handle failure
• Level 1: Mirroring
  • For each block, have copies on some other drive

    • Disk 0	Disk 1	Disk 2	Disk 3
      0	0	1	1
      2	2	3	3

  • More advanced failure model:
    • Block could become corrupt
    • Solutions
      • have > 2 copies, vote
      • Checksum
  • Good:
    • Performance (1 logical write => 2 physical write)
    • Tolerate failure
  • Bad:
    • Capacity (1/2 for 2 way mirror)
• Level 4: Parity
  • Bit level example, each row has even # of 1’s

  • Disk 0	Disk 1	Disk 2	Parity Disk
    0	1	0	1
    0	0	0	0

  • “Full stripe write”
    • Write: Disk 0,1,2 => RAID controller, compute parity
    • Do all writes in parallel
  • Random write: 1 block

    • Disk 0	Disk 1	Disk 2	Parity
      0	1	2	P0,1,2
      3	4	5	P3,4,5
      6	7	8	P6,7,8

    • How to write 4?
      • Approach #1 (Additive):
        • Read 3, 5
        • Compute Parity (Over 3, 4, 5)
        • Write 4, P3,4,5
      • Approach #2
        • Read old data
        • If different:
          • Read old parity
          • Compute (flip) the new parity
          • Write new data, new parity
        • 2 Reads + computation(free) + 2 writes
        • RAID 4: 1 Write => 4 I/Os
• Level 5
  • Stagger Parity

    Disk 0	Disk 1	Disk 2	Parity
    0	1	2	P0,1,2
    3	4	P3,4,5	5
    6	P6,7,8	7	8

  • Reduce write bottleneck
  • More parallel read

Mirroring Vs RAID - 5

• File System:
  • 2 abstractions
    • File:
      • array of bytes of some size, read or write or (grow or delete …)
        • FS doesn’t care about file contents
      • Has name: low-level (inode number)
    • Directory
      • List of files, directories
      • Map “human readable” name => low-level name