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Flash Memory based Storage

(CSE598D)

Thursday April 5, 2007

Youngjae Kim

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Disk Drive vs. Flash Memory

Read / Write

(+) Lost cost per bit

(-) Mechanical movement (SPM & VCM)(-) High power consumption (10-15W)(-) Heavy weight compared to flash

Read / Program / Erase

(+) Random Access(+) Non-volatile (+) Low Power Consumption (2W)

(-) Erase before Write(-) Erasing operation in the unit of block (not page)(-) Maximum # of erase operations per cell(-) High cost per bit

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MOS (Metal-Oxide Semiconductor) Memory Hierarchy

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History of Flash Memory

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NOR and NAND Flash Array

(a) NOR (b) NAND

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NOR and NAND Flash Array

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NAND Flash Memory – Program/Erase

• F-N tunneling– Give a higher voltage and electrons are trapped through gate into

floating gate transistor.

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Flash Memory Comparison

• NOR (Code Executable in Place like Memory)– Fast read and slow write

• NAND (Data-storage)– Fast write and lower cost

Flash Type Performance Application

Code

Storage

NOR

-Intel/Sharp

-AMD/Fujitsu/Toshiba

Important:

-High Random Access

-Byte Programming

Acceptable:

-Slow Programming

-Slow Erasing

Program Storage-Cellular Phone-DVD, Set TOP Box for BIOS

File

Storage

NAND

-Samsung/Thoshiba

Important:

-High Sped Programming

-High Speed Erasing

-High Speed Serial Read

Acceptable:

-Slow Random Access

Small form factor

-Digital Still Camera-Silicon Audio, PDA-Mass storage as Silicon Disk-Drive

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NAND Flash Non-Volatile Flash Cards

• Various Standard Memory Cards

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Functional Block Diagram for SAMSUNG K9K8G08U0M NAND Flash

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Array Organization for SAMSUNG K9K8G08U0M NAND Flash

• Block: Erasing Unit

• Page: Addressable Unit

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NAND Flash Technology

http://www.samsung.com/Products/Semiconductor/NANDFlash/index.htm

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Comparison for Different Memory Types

Design and evaluation of the compressed flash translation layer for high-speed and large-scale flash memory storages Proc. SoC Design Conference, pp. 740-745, September, 2003

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Outline

• Flash Memory Technology– NAND vs. NOR

• Block Mapping Schemes– Emulating Disk with Flash Memory

• Garbage Collection

• Hybrid Hard Drives– Window Vista

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NAND Type Flash Memory

• Operation – Read / Write

• Page unit (Size of a page = Size of a sector (512B) in hard drive)

– Erase • Block unit (A set of pages)

• Characteristics – Not in-place update

• Erase an entire erase block for in-place update of page

Original block

Free block

1. Update page 0in free block

2. Copy the rest ofPages (1,2,3,4)

3. Obsolete originalblock

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Block-Mapping Technique (1/2)

• Emulate Block device (Disk-Drive) with Flash Memory – In traditional disk drive,

1. File system calls a device drive, requesting block read/write

2. Device driver stores the data and retrieve it from flash device

• Problems in Simple Linear Mapping – Lifetime shortening of flash memory by the limit of write operations

• 100,000 – 1,000,000 per cell

– High risk of data loss due to the size difference between file system data block and erase block unit of flash

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Block-Mapping Technique (2/2)

• Maximum number of write operation– Some data block may be written much more than others

• No problem in hard drive

• Operation time to the cell get slow down => wear and burn out

• Data loss risk from size difference between data block and erase unit in flash– Example

• Copy an entire unit (128KB) into RAM and modify 4KB while erasing the entire unit and write back.

• But, power loss– (128KB + 4KB) data loss

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Block-Mapping Idea (1/2)

• Maintain mapping table – Virtual Block # - Physical Flash Address (Sector)

• Update Process1. Do not overwrite the sector, instead, write to another free sector2. Update mapping table (+) Evenly distribute the wear of erase units(+) Fast write (because of not-erasing process)(+) Minimize data loss when power off (possibly revert to the previous state)

• Write Process1. Search for a free/erased sector2. Initially, all the bits for (Sector and Header) should be 1s.3. Clear free/used bit4. Write virtual block # into the header and then write data in the sector5. Clear prevalid/valid bit6. Clear valid/obsolete bit of previous sector

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Block-Mapping Idea (2/2)

• Power off during write operation(Case 1) If the power off occurs before new sector is set to be valid

then ignore the data written

(Case 2) Even if new sector is set valid, but if the power is off before the previous sector becomes obsolete, then both of them are valid.

• Select any one according to their versioning numbers

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Data Structure for Mapping

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Flash Translation Layer

• Fully emulate magnetic disks with flash memory– Support random-access

• Two features of flash memory– Erase before write– Erase unit size (block) is not the same as read/write size (page).

File System

Block Device Driver

HOST Flash Device

Controller

ROM RAM

Flash Memory

IDE, SCSI

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Page-Level Mapping

• Logical Sector Number to Physical Sector Number

• Limitation – Large SRAM => High Cost

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Block-Level Mapping

• Logical Block Number to Physical Block Number + Offset

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• Limitation – Involving extra flash memory operation with write requests

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Hybrid Approach (Page + Block)

1 2 3 4 4 3 4Write Trace

Page 1 2 3 4 4 3 4

Block 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

ReplaceBlock 1 2 3 4 3 4 4

Log-Block 1 2 3 4 4 3 4

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Garbage Collection

• To make spare for new and update blocks– Obsolete sectors must be reclaimed. – To reclaim a sector is done by erasing an entire unit.

(Reclamation operates on entire erase units.)

• Reclamation – In background (e.g., when CPU is idle)– On-demand (e.g., when no free sectors)

– Goal• Wear-leveling • Efficient reclamation

• Reclaiming process1. Select erase units for reuse 2. Copy valid sectors (within an erasing unit) 3. Update mapping table 4. Erase the reclaimed erase units and add them to sector-reserve

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Wear Leveling

• Limitation – Maximum number of erases/writes per cell (10K – 1M)

• Reliability of cell decreases (e.g., bad block).

• Wear Leveling– To evenly distribute the cell usages over the cells– Wear-Leveling versus Efficiency

• They are contradictory.

• For, example, erasing unit containing STATIC data

– For efficiency, it should not be reclaimed

because any storage is not free up.

– For wear-leveling, it should be reclaimed

because it reduces the wear of other units.

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Wear-Centric Reclamation (1/3) [Lofgren et al. 2000, 2003]

Sector: Free

Sector: Valid

Sector: Valid

Sector: Valid

Counter

Sector: Free

Sector: Free

Sector: Free

Sector: Free

Counter

Sector: Valid

Sector: Free

Sector: Invalid

Sector: Valid

Counter

Most Worn-Out Unit (Reclaimed)

Least Worn-Out Unit

Spare Unit

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Spare Unit

• Using an erase counter of erase unit 1. When the most worn-out unit is

reclaimed, its counter is compared to that of the least worn-out unit.

2. If greater than threshold (e.g.,15,000), the contents of the most worn out unit are copied to the least worn out unit.And the most worn-out unit becomes spare.

3. Otherwise, just keep going as it does

• Wear-leveling– Moving static blocks to worn-out

sectors– Usually sector with static data is

least-worn out unit

1

Flash

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Wear-Centric Reclamation (2/3) [Jou and Jeppesen III 1996]

Sector: Free

Sector: Valid

Sector: Valid

Sector: Valid

Sector: Free

Sector: Free

Sector: Free

Sector: Free

Reclaimed Unit 1

Free Unit 3

Sector: Free

Sector: Valid

Sector: Valid

Sector: Valid

Sector: Free

Sector: Free

Sector: Free

Sector: Free

Reclaimed Unit 0

1Free Unit

Priority queue sorted by wear

U1U02

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• Using the wear (number of erasure)– The valid contents of erase unit

reclaimed are copied to another unit.– But, the unit is not erased

immediately– It’s marked as erasure unit and added

to queue of erase candidate (RAM)– The queue is sorted by wear– Whenever system needs a free unit

and the unit with least wear is erased

Flash

RAM

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Other Wear-Centric Reclamations (3/3)

• Using erase latencies [Han 2000]– Erase latency increases with wear. – Erase times are used to rank erase unit by wear. – It avoids to store erase counters.

• Randomized wear-leveling [Woodhouse 2001]– Every 1000th reclamation, a unit containing only valid data is

selected. – Pros and Cons

• (+) Moving static data from units with little wear to units with more wear

• (-) Extreme wear imbalance can occur. (e.g., a little worn-out unit with invalid data many never be reclaimed.)

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Wear-Leveling with Efficient Reclamation (1/2)

• Using a weighted benefit/cost [Kawaguchi et al. 1995]– Benefit: the amount of invalid space in the unit– Cost: the need to read the valid data and to write back

elsewhere– Weight: the age of the block, time since last invalidation

• Large weight → the remaining valid data are relatively static.

benefitcost X weight

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Wear-Leveling with Efficient Reclamation (2/2)

• Using Hot block and Cold block [Kawaguchi et al. 1995]– Cold block: Block allocated with low wear level– Hot block: Block with high wear level– Observation

• Units with dynamic data tend to be almost empty upon reclamation.

• Static units do not need to be reclaimed at all.

Flash

Cold Blocks Hot Blocks

1

Free Blocks 2

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Hybrid Hard Disk (HDD)

• Seagate’s ReadyDrive– HDD Prototype of Samsung & Seagate for Labtop (WinHEC conf.

2006)– 128 MB NAND Flash Memory in hard disk

• Store frequently accessed sectors of data for quick reads(e.g., FAT table)

– Flash is used to make less frequent disk power down and up. – Advantages

• Reliability, Power-Efficient, and Improved Performance

• Experiment– Run Office Applications– Spun up every three and four minutes– 10% power saving

http://www.extremetech.com/article2/0,1697,1966806,00.asp (May 24, 2006)

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Seagate’s 5400 RPM Hybrid Hard Drive

• 160 GB of regular perpendicular (PMR) space

• 256 KB flash memory

• It is for Windows Vista in Q1 2007.

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NAND Flash: Possible to replace the existing hard disks?

• This is suitable for mobile device. – Mobile device (e.g., Portable Digital Player, Cell Phone, etc.)

• Most applications are media (audio, video, etc.)

• Reads are dominate rather than writes.

• How about for server disk?– High cost/Large capacity for NAND flash, compared to traditional

disk• 64GB flash disk (Samsung, 2006) vs. 300GB Seagate Cheetah 15K.5

– Low reliability of data because of wear-out • Many writes could wear out the cells.

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FLASHCACHE [HCSS’94]

• DRAM Management

– LRU block replacement

• Flash Management

– Segment = A set of blocks/Erasing unit

– Segment list (Free/Clean/Dirty)

– Segment replacement (FIFO or LRU)

• Disk Management

– Power management by spin up/down

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eNVy [ASPLOS’94]

<Diagram of eNVy in a Host>

<Diagram of eNVy Architecture>

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eNVy [ASPLOS’94]

<Copy on Write: Atomic operation>

• Write operation– Flash memory: Copy-on-write operation

– SRAM is used as a Write Buffer for fast writes on flash.

– Page replacement on SRAM: FIFO

• Page-mapping (logical to physical addresses) on SRAM

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NVCache [MASCOTS’06]

• To reduce the power consumption of disk

• NVCache– To reduce disk power consumption by combining adaptive disk

spin-down algorithm – To extend spin-down periods by undertaking in NVCache

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Non-volatile Memory File Systems

• JFFS2 (Journaling Flash File System)– Built-in linux kernel after

2.4.X

– JFFS1 (1999) → JFFS2 (2001) → JFFS3 (on-going)

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Non-volatile Memory File Systems

• Using NVRAM at file system level

– Conquest file system [USENIX’02]• Persistent RAM (a sort of NVRAM)

• NVRAM stores “metadata”, “small files”, “executables”, and “shared libraries”.

– HeRMES file system [HotOS’01]• Magnetic RAM (a sort of NVRAM)

• NVRAM stores “metadata” and “small data / a few of first blocks”.– (+) reduce metadata overhead for writes/reads to improve performance

• NVRAM is use for write cache.– (+) enhance write performance (by buffering and reordering writes)