1 / 25

Lecture 24 Disk IO and RAID

CS 15-447: Computer Architecture. Lecture 24 Disk IO and RAID. November 12, 2007 Nael Abu-Ghazaleh naelag@cmu.edu http://www.qatar.cmu.edu/~msakr/15447-f08. Interfacing Processor with peripherals. Processor. L1 cache Instrs. L1 cache data. L2 Cache. Front side bus, aka system bus.

nestorl
Download Presentation

Lecture 24 Disk IO and RAID

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. CS 15-447: Computer Architecture Lecture 24Disk IO and RAID November 12, 2007 Nael Abu-Ghazaleh naelag@cmu.edu http://www.qatar.cmu.edu/~msakr/15447-f08

  2. Interfacing Processor with peripherals Processor L1 cache Instrs. L1 cache data L2 Cache Front side bus, aka system bus memory bus main memory bus interface I/O bridge To I/O

  3. Another view

  4. Disk Access • Seek: position head over the proper track(5 to 15 ms. avg.) • Rotate: wait for desired sector(.5 / RPM). RPM 5400—15,000 currently • Transfer: get the data(30-100Mbytes/sec)

  5. Manufacturing Advantages of Disk Arrays Disk Product Families Conventional: 4 disk designs 3.5” 5.25” 10” 14” High End Low End Disk Array: 1 disk design 3.5”

  6. RAID: Redundant Array of Inexpensive Disks • RAID 0: Striping (misnomer: non-redundant) • RAID 1: Mirroring • RAID 2: Striping + Error Correction • RAID 3: Bit striping + Parity Disk • RAID 4: Block striping + Parity Disk • RAID 5: Block striping + Distributed Parity • RAID 6: multiple parity checks

  7. Non-Redundant Array • Striped: write sequential blocks across disk array • High performance • Poor reliability:MTTFArray = MTTFDisk / NMTTFDisk = 50,000 hours (6 years)N = 70 DisksMTTFArray= 700 hours (1 month) Odd Blocks Even Blocks

  8. Redundant Arrays of Disks • Files are "striped" across multiple spindles • Redundancy yields high data availability • When disks fail, contents are reconstructed from data redundantly stored in the array • High reliability comes at a cost: • Reduced storage capacity • Lower performance

  9. RAID 1: Mirroring • Each disk is fully duplicated onto its “shadow”  very high availability • Bandwidth sacrifice on writes:Logical write = two physical writes • Reads may be optimized • Most expensive solution: 100% capacity overhead Used in high I/O rate , high availability environments

  10. RAID 3: bit striping + parity • A parity bit for every bit in the striped data • Parity is relatively easy to compute • How does it perform for small reads/writes?

  11. Redundant Arrays of Disks RAID 3: Parity Disk 10010011 11001101 10010011 . . . P logical record 1 0 0 1 0 0 1 1 1 1 0 0 1 1 0 1 1 0 0 1 0 0 1 1 0 0 1 1 0 0 0 0 Striped physical records • Parity computed across recovery group to protect against hard disk failures 33% capacity cost for parity in this configuration wider arrays reduce capacity costs, decrease expected availability, increase reconstruction time • Arms logically synchronized, spindles rotationally synchronized logically a single high capacity, high transfer rate disk Targeted for high bandwidth applications: Scientific, Image Processing

  12. RAID 4 (Block interleaved parity)

  13. Redundant Arrays of Disks RAID 5+: High I/O Rate Parity Increasing Logical Disk Addresses D0 D1 D2 D3 P A logical write becomes four physical I/Os Independent writes possible because of interleaved parity Reed-Solomon Codes ("Q") for protection during reconstruction D4 D5 D6 P D7 D8 D9 P D10 D11 D12 P D13 D14 D15 Stripe P D16 D17 D18 D19 Targeted for mixed applications Stripe Unit D20 D21 D22 D23 P . . . . . . . . . . . . . . . Disk Columns

  14. Nested RAID levels • RAID 01 and 10 combine mirroring and striping • Combine high performance (striping) and reliability (mirroring) • Get reliability without having to compute parities: higher performance and less complex controller • RAID 05 and 50 (also called 53)

  15. Operating System can help (1) Reducing access time • Disk defragmentation: why does that work? • Disk scheduling: operating system can reorder requests • How does it work? Reduce seek time • Example: Mean seek distance first, Elevator algorithm, Typewriter algorithm • Lets do an example • Log structured file systems

  16. Log structured file systems • Idea: most reads to disk are serviced from cache – locality! • But what about writes?  they have to go to disk; if system crashes, we the file system is compromised • How can we make updates perform better: • Save them in a log (sequentially) instead of their original location; why does that help? • Tricky to manage

  17. Operating System can help (2) Reliability • RAIDs are reliable to disk failures, not CPU failures/software bugs • If the cpu writes corrupt data to all redundant disks, what can we do? • Backups • Reliability in the operating system

  18. How are files allocated on disk? • Index block, has pointers to the other blocks in the file • Alternatives: linked allocation • Data and meta data both stored on disk • What do we do for bigger files?

  19. Unix Inodes

  20. Disk reliability • Any update to disk, changes both data and meta data • requires several writes • Operating system may reorder them as we saw • What happens if there is a crash? • Lets look at examples • Solution: journaling file system • Update journal before updating filesystem

  21. Flash Memory • Emerging technology for non-volatile storage – competitor to hard disks, especially for embedded market • Can be used as cache for the disk (much larger than RAM disks for the same price, and persistent) • Floating gate transistors: semi-conductor technology (like microprocessors and memory) – we know how to build them big (or small!) and cheap • Faster, lower power than disk drives • ...but still more expensive, and has some limitations • Two types of flash memory: NAND and NOR

  22. NOR Flash • NOR accessed like regular memory and has faster read time • Used for executables/firmware that dont need to change often (PDAs, cellphones, etc.. code) – Can be executed in place • bad write/erase performance (2 seconds to erase a block!) • bad wear properties (100,000 writes average lifetime)

  23. NAND Flash • Accessed like a block device (like a disk drive) • Higher density, lower cost • Faster write/erase time; longer write life expectancy • Well suited for cameras, mp3 players, USB drives... • Less reliable than NOR (requires error correction codes)

  24. Different properties from Disks • Flash memory has quite different properties from disks – Emphasis on seek time gone • Needs to erase a segment before writing (small writes are expensive!) • Slow...(especially NOR erase/write and NAND random access reads) • Must be done in large segments (10s of KBytes) • Can only be rewritten a limited number of times

  25. Summary of Flash circa. 2006

More Related