Post on 27-Jun-2020
transcript
Solid State Drives
By:Kevin AlderferMichael Bova
Thursday, November 10, 11
Overview
• SSD Basics
• HDD vs. SSD
• History of the SSD
• Flash Memory• NOR Type• NAND Type
• SLC• MLC
• Problems
• Today’s Solutions
• The Future of Data Storage
Thursday, November 10, 11
The Basics
• No Moving Parts
• Data stored in non-volatile flash memory chips
• Charge carriers are stored completely within the solid material
• No magnetism involved
Thursday, November 10, 11
HDD vs. SSDThursday, November 10, 11
HDD vs. SSD SpecsAttribute SSD HDD
Defragmentation No benefit Often required
Sound No sound Moving parts will make noise
Mechanical Breakdowns No moving parts All moving parts have a chance of
failure
Environment Not impacted by shock, altitude, or vibration Susceptible to these factors
Magnetism No impact Magnetic surges can alter saved data
Parallel Operation
Some controllers can have multiple chips reading and writing
different data simultaneously
Have multiple heads but are all connected. Can only write one thing at
a time.
Write Longevity
Limited writes (1-5 million). Software controllers manage this and can help SSDs last decades.
DRAM SSDs aren’t limited.
Unlimited writes, but eventual mechanical failure
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HDD vs. SSD Specs (cont.)
Attribute SSD HDDRandom AccessTime
.1ms 5-10msmove heads and rotation
Read Speed ~250 MB/s ~100 MB/s
Write Speed ~250 MB/s ~60 MB/s
Encryption Must erase data before overwriting Can directly overwrite data
Cost NAND SSDs ~ $.90-2.00 per GB ~ $.05/GB for 3.5” & $.10/GB for 2.5”
Storage Current Max: 2TBTypical: 64-256GB
Current Max: 2-3TBTypical: 500GB-1TB
PowerFlash based: 1/2 - 1/3 power of
HDDDRAM based: same as HDD
High performance: 12-18 wattsLaptop drives: 2 watts
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History of the SSD• Intel 1978 - Electrically Erasable Programmable Read Only Memory (EEPROM)
• Floating-gate transistors to store bits• About performance of RAM without volatility• Full disk system about speed of hard disk• Not much faster than HDD, but consumed 100 times the power• Much more expensive than HDD
• Toshiba 1980 - NOR Flash Memory
• Used much less power than EEPROM
• Allowed for more read/write cycles before burn-out
• 1989 - NAND flash
• Densely packed “pages”
• Much cheaper than NOR
• Reads have a much higher bit error rate, however
• MLC NAND flash
• Each cell of MLC has at least four states (whereas SLC has two) capable of storing at least two bits.
• This requires much finer, more precise measurement, and therefore results in a higher probability of error.
• Cheaper than SLC, but slower and more error-prone. As technology increases, more bits can be stored by MLC
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Flash Memory
A large Vg bumps up electrons onto Floating Gate from inversion layer
Electrons closer to Drain have more momentumEnough energy can bump them into Silicon atoms
The number of electrons on the Floating Gate affects Vt
Vt measured to determine state of cell
Limited lifespan due to Si degradationThursday, November 10, 11
NOR Characteristics
• Intel- 1980
• Traditionally used in portable electronics
• Cells connected in parallel, allowing for random access
• Low density with high read speeds
• Slow write: Blocks must be written with zeros before they can be erased and rewritten.
• Good for code execution (XIP- eXecute In Place)
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NAND Characteristics• Toshiba- 1989
• Very small cell size at the cost of cell parallelism (Resulting in indirect access)
• Eight memory transistors connected in series
• Replacing NOR due to faster write/erase, higher density, and lower cost-per-bit (less expensive)
• Write/erase whole blocks at a time: Much faster than NOR.
• Good for storage purposes
• Consumes less power
• Recent Density: 32GB on MicroSD card the size of a fingernail
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NAND vs. NOR: A ComparisonThursday, November 10, 11
Single-Level Cell
0 - Programmed
1 - Erased
Represents 1 bit
High performance and long term reliability
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Multi-level Cell
Generally Represents 2 bits
Must have rigidly controlled programmingPrecise amounts of charge storedPrecise Vt reading
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SLC vs MLC Specs
SLC MLCMLC
Density 16Mbit 32Mbit 64Mbit
Read Speed 100ns 120ns 150ns
Block Size 64Kbyte 128Kbyte128Kbyte
Architecture x8 x8 / x16x8 / x16
Endurance 100,000 cycles 10,000 cycles10,000 cycles
Operating Temperature Industrial
Commercial Commercial
Thursday, November 10, 11
SLC vs. MLC Overview
SLC MLC
High Density
Low Cost per Bit
Endurance
Operating Temperature Range
Low Power Consumption
Write/Erase Speeds
Write/Erase Endurance
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Problems
• Lifetime• Algorithms today keep track of block’s write count, and remaps the logic sectors of
block about to fail to new blocks.
• With one extra GB of surplus NAND flash, a single logical block could be written every second for several years without failing.
• Therefore, it is now very difficult to wear out an SSD unless you are very committed to doing so.
• Random-Write-Hole (RWH)• I/O Operations/sec slows from 1000s to 10s
• Heavily-polled dies burdened down by a gridlock of huge writes from small write requests, because it’s desirable to write full blocks
• Read Modify Write cycle (RMW)• SSD will perform RMW cycle when writing to the location of a formerly “deleted” file
• This adds an extra cycle every write once the drive is filled
• Taken care of by TRIM
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TRIM• Definition: A software command that allows an
Operating System to inform a SSD which blocks of data are invalid due to user or OS generated erases
• Enables SSD to handle garbage collection and free up space
• Necessary since SSDs operate vastly differently than HDDs at a low level
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RWH Fixes
• Sandisk: nCache• Software write buffer• Small section on disk (320MB for 64GB SSD)• Queues up small write and cleared out during idle moment• May not have idle moment for a long time, so won’t allow sustained
high levels of writing
• Intel: Sector Remap• Pools a number of small writes, in multiples of page size, on one block• Rewrites sector map table to combine these pages into a logical sector• X25-M, in G1 and G2 models, has the best RWH solution today
• G1 and G2 models have new firmware to fix fragmentation issue in earlier models
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Into the Future• SSDs will continue to improve in lifespan and cost.
• Holographic disks (Being developed by GE) utilize multiple layers of solid material to store exponentially more data than current single layer disks.
• Molecular memory: Using organic molecules instead of silicon may become a possibility in the future. The size of such molecules is in the order of trillions of times smaller than the silicon transistors used today.
• Bacteria: Bio-genetically encoded DNA.
• Quantum Computing: Storing data in the properties of quantum mechanical systems (i.e. an electron’s spin)
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THE END
Thursday, November 10, 11