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Solid%20State%20Drive

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Solid State Drive Feb 15 NAND Flash Memory Main storage component of Solid State Drive (SSD) USB Drive, cell phone, touch pad Solid State Drive (nowadays) Solid ... – PowerPoint PPT presentation

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Title: Solid%20State%20Drive


1
Solid State Drive
  • Feb 15

2
(No Transcript)
3
NAND Flash Memory
  • Main storage component of Solid State Drive (SSD)
  • USB Drive, cell phone, touch pad

4
Solid State Drive (nowadays)
5
Solid State Drive (SSD) architecture
6
Comparison
7
Comparison
Attribute SSD HDD
Random access time 0.1 ms 5-10 ms
Bandwidth 100-500 MB/s 100 MB/s sequential
Price/GB 0.9-2 0.1
Size Up to 2TB, 250GB common 4TB
Power consumption 5 watts Up to 20 watts
Read/write symmetry No Yes
Noise No Yes (spin, rotate)
8
Characteristics of SSD
  • High reliability (no moving parts)
  • Small form factor
  • Less noise
  • Read/Write latency w.r.t. HDD
  • No seek latency
  • Out-of-place update
  • Invalidate old version, and write new version
    somewhere else
  • Invalidated version becomes garbage
  • HDD, in-place update
  • Garbage collection (GC)
  • Erase at the unit of block
  • LBN to PBN mapping (FTL)
  • Emulate a block device
  • Erase cycle limit (endurance)
  • Read latency 25 us
  • Write latency 0.25 ms
  • Erase latency 2.5 ms

9
  • Advantages of NAND SSD
  • Fast random read (25 us)
  • Energy efficiency
  • High reliability (no moving parts)
  • Small form factor
  • Less noise
  • Widely deployed in high-end laptops
  • Macbook air, ThinkPad X series, touch pad
  • Increasingly deployed in enterprise environment
    either as a secondary cache or main storage

10
  • Disadvantages of SSD
  • Garbage collection (GC) problem of SSD
  • Stemmed from the out-of-place update
    characteristics
  • Update requests invalidate old version of pages
    and then write new version of these pages to a
    new place
  • Copy valid data to somewhere else (increasing
    number of IOs)
  • Garbage collection is periodically started to
    erase victim blocks and copy valid pages to the
    free blocks (slow erase 10xW,100xR)
  • Blocks in the SSD have a limited number of erase
    cycles
  • 100,000 for Single Level Chip (SLC), 5,000-10,000
    for Multiple Level Chip (MLC), can be as low as
    3,000
  • May be quickly worn out in enterprise environment
  • Performance is very unpredictable
  • Due to unpredictable triggering of the
    time-consuming GC process

11
Drive read performance
12
Flash Translation Layer (FTL)
  • Page-mapping
  • Block-mapping
  • Log-block-mapping

13
Flash Translation Layer (FTL)
  • Page-mapping
  • Mapping any Logical Block Address (LBA) to any
    Physical Page Address (LPA)
  • Require huge RAM to store the mapping entries
  • 512GB SSD requires 4GB RAM to store the mapping
    table
  • Each mapping entry is 16 Byte, each page size is
    2 KB

14
Flash Translation Layer (FTL)
  • Block-mapping
  • Map logical address to fixed offset of the
    physical address by taking module
  • LBNN, N is the number of pages in each block
  • LBN with the same offset may conflict, high
    overhead

15
Flash Translation Layer (FTL)
  • Log-block-mapping

16
Issues about deployment
  • Mixing read and write degrades the performance
  • Bandwidth drops as more data is being written
  • Garbage collection
  • The performance becomes worse in enterprise
    environment
  • Bandwidth can drop to 20 of the bandwidth of the
    datasheets
  • Endurance
  • Wear out in 23 days in enterprise environment
  • MLC can only wear

17
Mixed writes and reads
18
Improve write performance
  • OS write caching
  • Flash specific file system
  • Drive Write Caching
  • Multiple concurrent erase blocks
  • Expensive 200400 /GB

19
  • Flash File System
  • ZFS, has be optimized
  • JFFS2
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