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Integrating NAND Flash Devices onto Servers

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Cost of power and cooling contributes to significant portion of operating cost ... in the read disk cache that may only evict Flash blocks and pages on read misses. ... – PowerPoint PPT presentation

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Title: Integrating NAND Flash Devices onto Servers


1
Integrating NAND Flash Devices onto Servers
  • By David Roberts, Taeho Kgil and David Mudge
  • Presented by
  • Shivananda Reddy

2
AIM



  • Computer Servers

3
Motivation
  • Cost of power and cooling contributes to
    significant portion of operating cost of the data
    centre.
  • System memory power (DRAM) and disk power
    contribute as mush as 50 to the total power.

4
Contributions
  • Analysis of current and potential Flash usage
    models for servers.
  • Argue that the extended system memory model is
    the best usage model to reduce data center
    energy.
  • Two architectural modifications to improve
    NAND-based disk caches
  • Splitting Flash based disk caches into read and
    write regions.
  • Programmable Flash memory controller.

5
Questions ?
6
Outline
  • NAND Flash overview
  • NAND Flash usage models in a server
  • Architecture of the Flash-based disk cache
  • Architecture of the Flash memory controller
  • Operation and dynamic reconfiguration
  • Methodology
  • Results
  • Conclusion

7
NAND Flash overview
  • Flash page 2KB, Flash block 128KB (64 Flash
    Pages).
  • Random Flash reads and writes are performed on a
    page basis.
  • Flash erasures are performed per block.
  • Flash must perform an erase on a block before it
    can write to a page belonging that block.
  • SLC/MLC dual mode Flash is used.

8
NAND Flash models in a server
  • Extended system memory usage model A NAND Flash
    memory module is connected to the current system
    memory interface.
  • Storage accelerator usage model A NAND Flash PCI
    express card is connected to the PCI express
    interface.
  • Storage device usage model A Solid State Drive
    (SSD) replaces or augments the hard disk drive.
    Example SATA SSD.

9
Extended system memory
  • This presents Flash as a part of the system
    memory.
  • It addresses rising power contribution of DRAM.
  • Electrical constraints limiting the integration
    of more system memory.
  • It requires modification to the OS kernel.
  • Kernel memory manager needs to be aware of unique
    organization and behavior of Flash.
  • Flash reliability management can be performed by
    the kernel memory with the assistance of the
    Flash controller.

10
Storage accelerator
  • This model allows the server application to
    manage Flash directly as a cache that stores
    frequently accessed code and data.
  • It reduces the overall disk power by reducing the
    number of accesses to the hard disk drive.
  • Flash management is distributed across the user
    application, device driver stack and the Flash
    PCI express card Firmware.

11
Storage Device
  • Solid State Drive (SSD) replacing hard disk
    drive.
  • Improves latency and throughput to disk and
    reduces overall disk power consumption.
  • Requires modification in the kernel and a complex
    Flash device controller for Flash reliability
    management.
  • A customized file system is required to fully
    exploit the benefits of Flash.
  • Ties to non-Flash aware features in a hard disk
    drive interface protocol such as SATA.

12
Comparison
13
Flash-based disk cache (Arch)
14
Flash-based disk cache (Arch)
  • Two level disk cache, small DRAM in front of a
    dense Flash.
  • DRAM acts as a cache for the Flash.
  • Flash memory controller is required for
    reliability management.
  • MLC NAND Flash and is capable of switching from
    SLC mode. (Flex-OneNAND and Cho)
  • Variable strength ECC is used to improve
    reliability.
  • Additional data structures to manage the Flash
    blocks and pages.

15
Operating System Support
  • Flash cache hash table (FCHT) stores the
    mapping between addresses on disk and Flash.
  • Flash page status table (FPST) stores error
    correction code (ECC) strength, SLC/MLC mode and
    a saturating access counter. It also stores a
    valid bit field.
  • Flash block status table (FBST) records the
    number of erase operations performed on each
    block as well as the degree of wear out.
  • Flash global status table (FGST) tracks average
    latency and miss rate of the Flash based disk
    cache.

16
Splitting Flash into Read and Write
  • Out-of-place writes degrade the performance of a
    Flash-based disk cache.
  • Increases the garbage collection (GC) overhead.
  • Increases the number of overall disk cache
    misses.
  • Read critical Flash blocks are located in the
    read disk cache that may only evict Flash blocks
    and pages on read misses.
  • Write disk cache captures all writes to the Flash
    based disk cache and performs out-of-place
    writes.
  • Each read and write cache manages its own set of
    Flash blocks and LRU policy.

17
Splitting Flash into Read and Write
18
Flash memory controller
19
Flash based disk cache access
20
Methodology
  • Evaluated the Flash memory controller and Flash
    device using a full system simulator called M5.
  • Scaled the benchmarks, system memory size, Flash
    size and disk drive size to run on simulation
    infrastructure.
  • To measure power and performance dbt2 (OLTP) and
    SPECWeb99.

21
Results
  • System memory and disk energy efficiency

22
Results
  • Impact of BCH code strength on system performance.

23
Results
  • Flash lifetime

24
Conclusion
  • Flash reduces power consumption in system
    memories and disk drives while improving the
    overall throughput.
  • Integrating Flash on servers reduce the operating
    cost of a server platform.
  • Proposed architecture with reliability support
    greatly improves Flash lifetime.

25
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