Solid%20State%20Drive

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Solid State Drive

• Feb 15

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(No Transcript)
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NAND Flash Memory

• Main storage component of Solid State Drive (SSD)
• USB Drive, cell phone, touch pad

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Solid State Drive (nowadays)
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Solid State Drive (SSD) architecture
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Comparison
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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)
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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

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• 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

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• 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

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Drive read performance
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Flash Translation Layer (FTL)

• Page-mapping
• Block-mapping
• Log-block-mapping

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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

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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

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Flash Translation Layer (FTL)

• Log-block-mapping

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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

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Mixed writes and reads
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Improve write performance

• OS write caching
• Flash specific file system
• Drive Write Caching
• Multiple concurrent erase blocks
• Expensive 200400 /GB

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