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-FTL: A Memory-EfficientFlash Translation Layer

Supporting MultipleMapping Granularities

Yong-Goo Lee

Computer Architecture Laboratory

Korea Advanced Institute of Science and Technology (KAIST)

South Korea

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Advantages of NAND Flash Memory

Non-volatility

Short read/write latency

Low power consumption

Small size and light weight

Solid state reliability

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NAND Flash Memory Characteristics

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page A A

A unit of NAND flash memory operation

Page: a read/write unit (4KB)

Block: an erase unit (512KB)

Erase-before-write

A page once written cannot be overwritten until it is

erased

NAND flash memoryblock

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Flash Translation Layer

Legacy disk-based host systems cannot be

directly used with NAND flash memory

FTL translates a logical address used by

the host into a physical address in NANDflash memory

Two major factors affecting the

performance of FTL

Address mapping mechanism

Garbage collection policy

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FTL

NAND Flash memory

Host system(file system, device

driver)

Logicaladdress

Physicaladdress

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Motivation

6RAM usage

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Main Ideas of -FTL

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Workload Characteristics

Multiple mapping granularities

Reducing both the RAM usage and the overhead Using -Tree as a main data-structure

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Characteristic(write requests)

Example Appropriate mappinggranularity

Small &Random

File system meta dataTemporary Internet files

Fine-grained

Large & SequentialMultimedia filesInstalled applications Coarse-grained

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-Tree [D. Kang, EMSOFT2007] Handling any insertion/deletion/update operation of the tree

with only one flash write operation

By storing multiple nodes from the leaf to the root in a single page

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LRU

head

-Tree cache

cachemiss cache full

(dirty pagesonly)

A

B C

D E F

A

C

F

A

B C

D E F

A

B C

D E F F

A

B C C

D E F F

A A

B C C

D E F F

-Tree block

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Address Mapping

-FTL implements multiple mapping granularities Extent: A variable-sized mapping entry (a pair of key and record)

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Logical

blocks

Physical

blocks 5

0 4 5 6

100,4

105,1

104,1

106,2

Key

Record

-FTL mapping

example

The implementation of

mapping example

100101102103

104105106107

0123

4567

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Main Ideas of -FTL

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Garbage Collection

Victim selection policy

Having the largest # of invalid pages

Garbage collection information

Per-block invalid page counter

Maintaining the number of invalid pages in each

physical block

Bitmap information

Distinguishing valid pages from invalid pages

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5 25

Bitmap Information

Bitmap information is too large to be stored in RAM

We store the bitmap information in -tree

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0 4 5 6

100,4

105,1

104,1

106,2

Physicalblocks100101102103

104105106107

25 26

(0001)2

(1111)2

Key (bitmap)

Record

1 125 26

Key(mapping)

1

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Bitmap Cache

The -Tree cache is polluted by bitmap entries The cache hit ratio of extents severely impaired

Bitmap cache

A hash table structure

for buffering bitmap updates

Invalid extent Physically adjacent

invalid pages

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(Physical Page Number,Length)

Pointer

InvalidExtent

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Main Ideas of -FTL

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Logical Address Space Partitioning

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A global update

block

Many update blocks

for each partition

Logical addressspace

Logical addressspace

Separating hot data from cold data has a beneficial effect onthe performance of an FTL

Each part of the logical address space exhibits different

degree of hotness

Update block being charged for receiving data from incoming write requests

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Main Ideas of -FTL

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-FTL Design Summary

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Write

Bitmap

Cache-Tree

Cache

Partition 1

Partition 2

Partition 3

Partition 4-Tree

Free block list

Data

Extentupdate

Bitmapupdate

Bitmap updateflush

Cache

full

Cachemiss

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Evaluation Methodology

Trace-driven simulators for several FTLs Block-mapped: the log block, FAST, the super block

Page-mapped: DAC

6 traces 3 multimedia traces: PIC(8GB), MP3(8GB), MOV(8GB)

3 PC traces: WEB(32GB), GENERAL(32GB), SYSMARK(40GB)

The standard configurations

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Partition Size Bitmap cache size The portion ofextra blocks

256MB(512 logical blocks)

4KB for multimedia traces32KB for PC traces

3%

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The RAM Usage

Block-mapped FTL -FTL (A+B+C) Page-mappedFTL

8GB 64KB 16KB+44KB+4KB= 64KB

8MB

32GB 256KB 64KB+160KB+32KB= 256KB

32MB

40GB 320KB 80KB+208KB+32KB= 320KB

40MB

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(A) Per-block invalid page counter

(B) The -Tree cache size

(C) The bitmap cache size

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The Comparison of FTL Performance

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GENERAL (32GB) SYSMARK (40GB)

56.8%89.7%

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The Effect of Bitmap Cache Size

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WEB (32GB) PIC (8GB)

52.5% 33.3%

-Tree cacheoverhead

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The Effect of Partitioning

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WEB (32GB) GENERAL (32GB)

23.6% 39.6%

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Conclusion

-FTL Small and fixed RAM usage

As small as block-mapped FTLs

Low management overhead As low as DAC (a page-mapped FTL)

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Thank you for listening

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