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USENIX HotStorage ’15

Santa Clara, CA, USA, July 6~7, 2015

Suspend-aware Segment Cleaning

in Log-Structured File System

Dongil Park, Seungyong Cheon, Youjip Won

Hanyang University

Outline

Introduction

Log-structured File System and Segment Cleaning

Suspend Mode on Smartphone

Problem assessment

Suspend-aware Segment Cleaning

Design

Virtual Segment Cleaning

Utilization-based Segment Cleaning

Experiment

Conclusion

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 2

Introduction

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 3

• Flash memory based storage is widely used nowadays.

• Log-structured file system has been around for 20+ years.

• It has not been deployed on commercial devices.

• F2FS (Flash Friendly File System) is a log-structured file system

optimized for mobile device.

• F2FS is adopted in commercial mobile devices as their stock file

system.

• Motorola Droid family

• Motorola Moto X, Moto G, Moto E

• Google Nexus 9

Flash Friendly File System

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 4

F2FS shows great random write performance on flash memory.

more than 3ⅹ

source: C. Lee et al., FAST ‘15

sequential

write

sequential

read

random

write

random

read

Log-structured File System

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 5

In-place update file system

inode data

Log-structured file system

A C B CA B Disk

DiskA A B B C C A’ A’ Log

update(file A)

A’ A’

update(file A)

Log-structured File System

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 6

Segment: unit of disk write

Block: unit of write operation

Disk

Block and Segment

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 7

No data

Valid data

Invalid data

Free block

Valid block

Invalid block

Only free blocks

Valid & free blocks

No free block

Including invalid block

Free segment

Valid segment

Invalid segment

Writing in Log-structured File System

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 8

Writing data in append-only manner

A A A A

B B C C

XXXX

X X Y Y

Y Y Y Y

Y

Modify A

… … … …

… … … …

Y Z Z

… …

segment 1

(valid)

segment 2

(valid)

segment 0

(invalid)

A’ A’

A’ A’

X’ X’

Segment Cleaning

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 9

Segment cleaning

segment 1

(valid)

segment 2

(valid)

Y Y Y Y

Y

data data data data

data data data data

Y Z Z

data data

segment 0

(invalid)

A A A A

C C

XXXX

X

victim

segment

1. Select victim segment.

2. Migrate valid blocks.

3. Mark victim segment as free.

X

B

A’ A’

A’ A’ C’ C’B

Y Y

Segment Cleaning

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 10

Foreground segment cleaning: on-demand

Background segment cleaning: periodically

Two types of segment cleaning

Write

Segment

cleaning

request

time

completeI/O latency

Write

Segment

cleaning

time

request complete requestidle

period

Suspend Mode of Smartphone

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 11

Android adopts Suspend Mode to save battery

buffer

disk

task

Suspend Mode of Smartphone

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 12

Android adopts Suspend Mode to save battery

buffer

disk

task

Problem

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 13

Log-structured

File System

Log-structured file system needs

segment cleaningto generate free space

Android smartphone is

suspendedwhen screen is turned off

Does segment cleaning works well

when smartphone is suspended?

Androidvs.

Segment Cleaning and Suspend Mode

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 14

Power management

File system time

Background

segment

cleaning𝑆0 𝑆1 𝑆2 𝑆3 𝑆4 𝑆5 𝑆6 𝑆7

Suspend

Active

Segment Cleaning and Suspend Mode

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 15

Power management

File system time

Background

segment

cleaning𝑆0 𝑆1 𝑆2 𝑆3 𝑆4 𝑆5 𝑆6 𝑆7

Suspend

Active

Segment Cleaning and Suspend Mode

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Power management

File system time

Background

segment

cleaning𝑆0 𝑆1 𝑆2 𝑆3 𝑆4 𝑆5 𝑆6 𝑆7

Suspend

Active

CONFLICT!!

Observation

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 17

Active period in real setting

Average 5.6 sec.

Device may be in suspend state when

Background Segment Cleaning is activated.

0%

20%

40%

60%

80%

100%

user 1 user 2

Suspend Active

About 75%

Observation

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There is a large time slack

Active

Suspend

time

Average

1.8 sec.

Waiting for the wakeup events to be cleared

Design

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 19

Use the time between LCD off and entering suspend!

• CPU and I/O devices are still awaken

• Does not interfere with user and foreground apps

• Perform segment cleaning when suspending device

Virtual Segment Cleaning

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victim

segment

page cache

Y Y

BB

X’ X’

Y Y Y Y

Y

… … … …

… … … …

Y Z Z

… …A A A A

C C

XXXX

X X

A’ A’

A’ A’ C’ C’

Do not migrate valid block directly

Virtual Segment Cleaning

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victim

segment

page cache

Suspend routine start

Y YBB

X’ X’

Y Y Y Y

Y

… … … …

… … … …

Y Z Z

… …A A A A

C C

XXXX

X X

A’ A’

A’ A’ C’ C’

Do not migrate valid block directly

Mark page cache entry as dirty

Virtual Segment Cleaning

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 22

victim

segment

page cache

Call sys_sync()

Y Y

BBX’ X’

Y Y Y Y

Y

… … … …

… … … …

Y Z Z

… … A’ A’

A’ A’ C’ C’

Do not migrate valid block directly

Mark page cache entry as dirty

Virtual Segment Cleaning

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 23

time

LCD off sys_sync()

suspended∆

𝑉𝑆𝐶1 𝑉𝑆𝐶2 𝑉𝑆𝐶3

𝑡𝑖𝑑𝑙𝑒 𝑡𝑖𝑑𝑙𝑒

𝑉𝑆𝐶𝑖 : 𝑖𝑡ℎ Virtual Segment Cleaning

∆ : Time for processing other suspend routine

𝑡𝑖𝑑𝑙𝑒 : Idle period to throttle the amount of reclaimed segments

Too frequent segment cleaning may leads to the reduction of the NAND flash lifetime

Utilization-based Segment Cleaning

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 24

0

5

10

15

20

25

30

35

40

45

50

55

60

65

0 10 20 30 40 50 60 70 80 90 100

𝑝𝑖𝑛𝑣

(Invalid

blo

ck r

atio,

%)

𝑢 (File system utilization, %)

Threshold of file system utilization to activate SC

Idea threshold

Activate SC

Approximated threshold

𝑢: File system utilization 𝑝𝑖𝑛𝑣: Invalid block ratio

Utilization-based Segment Cleaning

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 25

300 𝑚𝑠 < 𝑡𝑖𝑑𝑙𝑒 < 900 𝑚𝑠

Segment cleaning is activated with threshold and idle period is 𝑡𝑖𝑑𝑙𝑒

Idle period between segment cleaning

• Reclaim 2 ~ 6 free segments at once

• Adaptive with file system utilization and invalid block ratio

• Using time slack between LCD off and entering suspend

Experiment

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 26

Motorola Moto G (KK 4.4.2, Linux 3.4)

Component Specification

AP Qualcomm Snapdragon 400

CPU Cortex-A7 1.2GHz Quad-core

RAM 1GB

Internal storage 8GB eMMC

File system F2FS (data), EXT4 (others)

Experiment

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 27

0

20

40

60

80

100

120

0 10 20 30 40 50 60

# o

f re

cla

imed s

egm

ents

time (min.)

Number of reclaimed segments

reclaimed 6× more segments

Original Segment cleaning, suspend on

Original Segment cleaning, suspend off

Suspend-aware Segment Cleaning, suspend on

Idle Period Model Verification

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Number of reclaimed segments vs. File system utilization

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

20 30 40 50 60 70 80 90 100

Avg. # o

f re

cla

imed s

egm

en

ts

Utilization (%)

• Vary the file system utilization and the fraction of invalid blocks

• Enter suspend mode 50 times

𝑝𝑖𝑛𝑣 = 25% 𝑝𝑖𝑛𝑣 = 50% 𝑝𝑖𝑛𝑣 = 75%

increased by file system utilization

increased by invalid block ratio

2.12.3

2.32.42.7

2.9 2.9

33.7 3.9

3.9

4

Ideal value

Conclusion

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 29

• Background segment cleaning of log-structured file system and

suspend feature of Android system directly conflict with each other.

• Suspend-aware Segment Cleaning seamlessly reclaims free

segments with suspend module of the smartphone

Suspend-aware Segment Cleaning claims

6× as many segments as

the stock background segment cleaning does.

Dongil Park et al. USENIX HotStorage’15, Santa Clara, CA, July 6~7 30