Advantage of NAND Flash memory› Low-power consumption, small size and high shock resistance› Mobile consumer devices

Convergence of consumer devices› Full Web browsing, e-mail service, general system(SSD, HHD)› Workloads are more complex and more diverse

Constraint of NAND Flash memory› Erase-before-write ,Asymmetry of I/O unit size and Erase unit size› Poor write performance› FTL(Flash Translation Layer)

Mapping logical page address to physical address

› Flash-aware buffer management scheme Reduce the number of write request sent to the flash memory

FTL(Flash Translation Layer)› Block-level mapping

Small table size, high migration cost

› Page-level mapping Large table size

› Hybrid mapping (log buffer-based FTL) Data blocks + Log blocks Merge operation Poor performance for random write pattern

Flash-aware buffer management scheme › Reduce the number of write requests sent to flash memory

In this paper› A novel buffer replacement policy which considers the log

buffer in flash memory

Log Buffer-Based FTL› BAST (1:1 log block mapping)

Write sequence(p8, p12, p1,p5,p9,p13) Merge operation for every write Log block thrashing

› FAST (1:N log block mapping) Log block L1 is associated with four data blocks (B0,B1,B2,B3) 16 page copies for L1 merge High block associativity

Flash-Aware Buffer Schemes› FAB

victim block has largest number of pages Evicts all pages of a block at a time Also, block thrashing problem FAB can evict the recently used page

› BPLRU (Block Padding Least Recently Used) Victim block is least recently used block Only switch merge Block level LRU list management

->Not-recently-used pages waste the space of buffer cache

REF Page Eviction1. Block-level page eviction2. Log buffer-aware victim selection3. Page-level recency consideration› REF scheme can reduce the block associativity of log block as well as

the number of block merge

p9 p13p0 p4p1 p5 Block associativity =

4

Block associativity = 2

Victim Block Selection› VB : set of victim blocks› VW : victim window› |VB|=2, size of VW = 75% = 6 pages

› REF evicts the victim pages only by the amount of the required free space

› The size of victim window should be selected carefully

Problem of BPLRU

BP-REF performs the block padding technique selectively› Block padding threshold

B0

B1

B2

P0 P4 P8

P1 P5 P9

P2 P6 P10

P3 P7 P11

L0 L1 •BAST•Write request P5•P0 is evicted•P1, P2, P3 is copied

MRU

P8 P4 P0 LRU

Buffer cache P0

P1

P2

P3

•Block padding threshold : 6•Switch merge of L0 is possible

I/O trace › Internet Explorer› MS-Office install› JPEG file copy

Total execution time› When VW is 75% and |VB| is 3, the Performance of REF is generally best› REF improves the execution times by20~30% over other schemes

Number of block merge

› REF reduces especially the number of Full merge

Improvement of BP-REF

› Optimum threshold value is 10% and 40% in the BAST and the FAST respectively

REF› Flash-aware page eviction technique› Evict only the pages of the victim block› The victim block remains unchanged as far as possible› REF shows better performance than LRU, FAB and BPLRU

BP-REF› Selective block padding› Reduce block padding overhead

Future work› Dynamic adaptation technique for sizes of victim window and victim

block