A New LSM-style Garbage Collection Scheme for ZNS SSDs · HotStorage'20 Embedded System Lab. A New...

Embedded System Lab.HotStorage'20

A New LSM-style Garbage Collection Schemefor ZNS SSDs

2020. 07. 13

Presentation by Choi, Gunhee

[email protected]

Gunhee Choi, Kwanghee Lee, Myunghoon Oh and Jongmoo Choi from Dankook University Jhuyeong Jhin and Yongseok Oh from SK hynix12th USENIX Workshop on Hot Topic in Storage and File System (HotStorage 20), 2020

Embedded System Lab.

Content

HotStorage'20 2

1. Zoned Namespace SSD

2. Motivation

3. LSM-ZGC Design

4. Evaluation

5. Conclusions

- Appendix


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3

Traditional SSD

Open-Channel SSD Key-Value SSD

Zoned Namespace SSD Optane SSD

• What are Next Generation SSDs?

vs.


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4

Traditional SSD

ü Benefits• Better performance and WAF by distributing different workloads into different zones

• Better isolation (IO Determinism)• Reduce DRAM usage and Over-provisioning area in SSDs

LBA space

NAND

LBA space

NAND

Zone 1 Zone 2 Zone 3 …

Zoned Namespace SSD

• What is ZNS SSD?


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5

Host Needs to handle zone controlsSequential write constraint

Host

ZNS SSD Controller

Zone

WritePointer

Writingprogress direction

Unable to writeuntil zone reset

Zone 1 Zone 2 Zone N…

Block deviceFTL absence

App 1 App 1

• What are the issues of ZNS SSD?

ü Sequential write constraint: writes need to be conducted in a sequential manner, like the SMR drives.

ü Host needs to control zones directly such as zone open, close, reset and zone garbage collection.


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2. Motivation

6

SK Hynix PrototypeZNS SSD

Ref : https://news.skhynix.co.kr/1915

• How much is the Zone Garbage Collection (hereafter ZGC) overhead?ü Using real ZNS SSD prototypeü Zone size: 1GB (note that the typical segment size in LFS is 2MB)

https://news.skhynix.co.kr/1915


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2. Motivation

7

Basic Zone Garbage Collection (Basic_ZGC)

MemoryZoneBitmap

Zone 0

…

Zone 100

…

Step 1.Select a candidate zone (Greedy or CB)


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2. Motivation

8

Basic Zone Garbage Collection

MemoryZoneBitmap

Zone 0

…

Zone 100

…


Step 2.Find out valid blocksusing a zone bitmap


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2. Motivation

9


MemoryZoneBitmap

Zone 0

…

Zone 100

…

Step 3.Read valid data in 4KB (or larger) I/O size




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2. Motivation

10


MemoryZoneBitmap

Zone 0

…

Zone 100

…




Step 4.Write data in 4KB (or larger) I/O size


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2. Motivation

11


MemoryZoneBitmap

Zone 0

…

Zone 100

…




Step 4.Write data in 4KB (or larger) I/O size

Step 5.Reset the selected zone


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2. Motivation

12

• Zone : 1GB• Block : 4KB

Observation 1: Zone garbage collection overhead


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2. Motivation

13

• Zone : 1GB• Block : 4KB

☞Motivation 1: reducing utilization of a candidate zone is indispensable

5 times!

18 times!

Observation 1: Zone garbage collection overhead


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2. Motivation

14

Observation 2: I/O size for Read/Write

• Another feature of ZNS SSD ü A zone is, in general, mapped into multiple channels/ways.

• Then, how about read/write data in a larger I/O size (e.g. 128KB)?


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2. Motivation

15

Observation 2: I/O size for Read/Write

11 Times!

☞Motivation 2: accessing in a larger I/O size is beneficial in ZNS SSDs


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3. LSM-ZGC Design

16

So, Our ideas are1) Make the utilization of a candidate zone low2) Access data in a larger I/O size

• How to access data in a larger I/O size? ü The coexistence of valid and invalid data makes it difficultü Read not only valid but also invalid data in a larger I/O size

• How to make the utilization of a candidate zone low? ü Traditional hot/cold separation is not applicable in ZNS SSDs since zone is quite bigü Employ the segment concept for finer-grained hot/cold separation


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3. LSM-ZGC Design

17

• Two management unitsü Zone: for garbage collection vs. Segment: for hot/cold separationü A zone is divided into multiple segments (1GB vs. 2MB in this study)

• Segment state and transition rule (refer to our paper for details)ü New data è C0ü During ZGC, survived data from C0

§ Data in a high utilized segment ( > thresholdcold): cold è C1C § Others: hot (or unknown) è C1H § Reasoning: spatial locality, also observed in previous studies such as F2FS (FAST’15),

Multi-stream (FAST’19), Key-range locality (FAST’20) ü During ZGC, survived data from C1C or C1H (second survived data)è C2


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3. LSM-ZGC Design

18

Zone 0

Zone 1

Zone 2

Zone N-1

C0_zone

LSM(Log Structured Merge) Zone GC

Zone 3

…

Zone N

ZoneBitmap

…

Step 1.Select a candidate zone (or zones, Greedy or CB)

C1H_zone

C1C_zone


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3. LSM-ZGC Design

19

Zone 0

Zone 1

Zone 2

Zone N-1

C0_zone

………

2MB 2MB

…

2MB


Zone 3

…

Zone N

ZoneBitmap

…


Step 2.Read all data in 128KB I/O size

C1H_zone

C1C_zone


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3. LSM-ZGC Design

20

Zone 0

Zone 1

Zone 2

Zone N-1

C0_zone

………

2MB 2MB

…

2MB


Zone 3

…

Zone N

ZoneBitmap

…



Step 3-1.Check valid data

Step 3-2.Identify Hot/Cold segment

C1H_zone

C1C_zone


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3. LSM-ZGC Design

21

Zone 0

Zone 1

Zone 2

Zone N-1

C0_zone

………

2MB 2MB

…

2MB


Zone 3

…

Zone N

ZoneBitmap

…

C1H_zone

C1C_zone


…

2MB


Step 4.Merge valid data only according to hot/cold

…

2MB

Hot Segment

Cold Segment




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3. LSM-ZGC Design

22

Zone 0

Zone 1

Zone 2

Zone N-1

C0_zone

………

2MB 2MB

…

2MB


Zone 3

…

Zone N

ZoneBitmap

…


C1H_zone

C1C_zone


…

2MB

Step 4.Merge valid data only according to hot/cold

…

2MB

Hot Segment

Cold Segment

Step 5.Write data in 128KB I/O size




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4. Evaluation

23

Experimental environment• Intel Core i7 (8 core)• 16GB DRAM• 1TB ZNS SSD• Size of Zone : 1GB

Average of 1.9 times

Garbage collection overhead: uniform update pattern

Max of 2.3 times


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4. Evaluation

24

Garbage collection overhead: skewed update pattern

Experimental environment• Intel Core i7 (8 core)• 16GB DRAM• 1TB ZNS SSD• Size of Zone : 1GB

Average of 1.4 timesMax of 1.6 times

Parameters• Workload: 70/30 hot/cold ratio• Threaholdcold : 0.8 • average utilization: x-axis

0

10

20

30

40

50

0.5 0.6 0.7 0.8 0.9Garb

age

colle

ctio

n tim

e (s

ec)

Utilization

LSM_ZGC Basic_ZGC


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4. Evaluation

25

Hot/Cold Separation

ü Without hot/cold separation

ü With hot/cold separation

Parameters• Workload: 70/30 hot/cold ratio• Threaholdcold : 0.8 • Average utilization: 0.6

0

50

100

150

200

250

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Count

Utilization

Garbage Collection Count : 100

0

50

100

150

200

250

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Coun

t

Utilization


020406080100120140160180

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Count

Utilization


0

50

100

150

200

250

300

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Count

Utilization


020406080100120140

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Count

Utilization


0

20

40

60

80

100

120

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Count

Utilization



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5. Conclusions

26

• Our contributions§ Observation: a zone garbage collection really matters§ Proposal: a new LSM-style zone garbage collection scheme§ Evaluation: real implementation based results

• Future work

§ We are currently extending F2FS on our ZNS SSD prototype§ Also, evaluating LSM ZGC under diverse workloads with different hot

/cold ratio, data size, initial placement and classification policies

Embedded System Lab.HotStorage'20

Thank You!

A New LSM-style Garbage Collection Schemefor ZNS SSDs

2020. 07. 13

Presentation by Choi, Gunhee

[email protected]

Gunhee Choi, Kwanghee Lee, Myunghoon Oh, Jhuyeong Jhin, Yongseok Oh, Jongmoo Choi12th USENIX Workshop on Hot Topic in Storage and File System (HotStorage 20), 2020

Questions?


HotStorage'20

6. Appendix

28

Performance comparison using multi-thread & Scalability

Non-linearScalability

Worker only: 40With LSM_ZGC : 45With Basic_ZGC : 53


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6. Appendix

29

Sensitive Analysis: various parameters

ü Effect of thresholdcold (initial utilization: 0.6)

ü Effect of initial utilization of a zone (thresholdcold : 0.8)

020406080100120140160180

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Coun

t

Utilization

Threshold : 0.6

020406080100120140160180

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Count

Utilization

Threshold : 0.8

0

20

40

60

80

100

120

140

160

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9Co

unt

Utilization

Initial Utilization : 0.8

0

50

100

150

200

250

300

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Coun

t

Utilization


0

50

100

150

200

250

300

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Coun

t

Utilization


020

406080

100120

140160

180

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Coun

t

Utilization


020406080100120140160

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Count

Utilization

Threshold : 0.9

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A New LSM-style Garbage Collection Scheme for ZNS SSDs · HotStorage'20 Embedded System Lab. A New...

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