Improving File System Performance of Mobile Storage Systems Using a Decoupled Defragmenter

Transcription

1 Improving File System Performance of Mobile Storage Systems Using a Decoupled Defragmenter Sangwook Shane Hahn *, Sungjin Lee, Cheng Ji, Li-Pin Chang +, Inhyuk Yee *, Liang Shi #, Chun Jason Xue and Jihong Kim * * Seoul National University Daegu Gyeongbuk Institute of Science and Technology (DGIST) City University of Hong Kong + National Chiao-Tung University # Chongqing University USENIX Annual Technical Conference, 2017

2 Outline Impact of File Fragmentation/Defragmentation Key Observations on Flash-based File Fragmentation Decoupled Fragmentation Dominant Impact of Logical Fragmentation Janusd: a Decoupled Defragmenter Experimental Results Conclusions 2

3 Gradual Performance Degradation on Smartphones Performance of Android smartphones gradually degrades as smartphones age Application launching time Galaxy S6 (new) Galaxy S6 (6 months used) Galaxy S6 (1 year used) Galaxy S6 (2 years used) Application launching times increase up to 3 times on 2-year used smartphones 3

4 Root Cause: File Fragmentation Q: Why does performance degrade? A: File fragmentation Application launching time Nexus 6 G5 Galaxy S6 Xperia Z3 16% files fragmented 22% files fragmented 27% files fragmented 34% files fragmented Defragmentation can improve the degraded performance by fragmentation 4

5 File Fragmentation Recurrence on Smartphones Average # of fragments per file Degree of file fragmentation Q: How often should we defrag smartphones? Xperia Z3 Galaxy S6 G5 Nexus 6 S6 (92%) N6 (83%) N5 (74%) S3 (63%) 0 day 1 day 3 days 7 days 14 days Days since defragmentation File fragmentation recurs even in a week User begins to experience performance degradation 5

6 Lifetime Impact of Frequent Defragmentation Q: Are there any side effects from frequent defragmentation? The amount of data copies by defragmentation (GB) day 3 days 7 days 14 days 0 Nexus N5 (74%) 6 N6 G5 (83%) Galaxy S6 (92%) S6 Xperia Z3 Z3 Weekly defragmentation can reduce the storage lifetime by more than 10% 6

7 Summary: Impact of File Fragmentation/Defragmentation Decoupled Defragmenter High Performance Short Lifetime High Performance Long Lifetime Low Performance Long Lifetime Defragmentation with near zero data copies is needed NAND Flash-based Storage File fragmentation in NAND flash-based storage is quite different from conventional one in HDD 1. Decoupled fragmentation 2. High overhead of logical fragmentation 7

8 Observation 1: Decoupled Fragmentation All data are stored using address indirection Logical address space Contiguous File A Fragmented File B HDD Physical address space Contiguous Sectors Fragmented Sectors Fragmentation at logical space and physical space Logical address space Contiguous File A NAND flash-based storage Address indirection Physical address space Physical fragmentation Contiguous File A Fragmented File B Evenly distributed data Unevenly distributed data High degree of I/O parallelism Low degree of I/O parallelism 8

9 Observation 1: Decoupled Fragmentation Logically fragmented but, physically contiguous Percentage (%) Logically contiguous but, physically fragmented Low None Medium High Less than 1% 1. There is no correlation between logical/physical fragmentation 2. Physical fragmentation rarely occurs 9

10 Observation 2: High Overhead of Logical Fragmentation Android Platform File System Block I/O Layer Device Driver Mobile Storage Q: How much the impact of logical/physical fragmentation on performance? I/O Execution Time (us) File System Block Layer Device Driver # of block I/Os increases None Logical fragmentation Low Medium High Degree of Logical Fragmentation I/O Execution Time (us) Logical fragmentation overhead overwhelms physical fragmentation overhead None Mobile Storage Low Physical fragmentation Medium High Degree of Physical Fragmentation 10

11 Solution for Decoupled Fragmentation Logical Defragmenter Janusd Physical Defragmenter Logical fragmentation Physical fragmentation Defrag logical fragmentation using address remapping without data copies Janus Defragmenter Improve the low degree of I/O parallelism Common case Contiguous File Fragmented File Logical fragmentation Rare case Contiguous File Fragmented File Physical fragmentation Application Launching Time Application Launching Time 11

12 Outline Impact of File Fragmentation/Defragmentation Key Observations on Flash-based File Fragmentation Decoupled Fragmentation Dominant Impact of Logical Fragmentation Janusd: a Decoupled Defragmenter Experimental Results Conclusions 12

13 Logical Defragmenter Physical Defragmenter Overview of Decoupled Defragmenter Decoupled Defragmenter (Janusd) e4defrag Detect logical fragmentation Firmware (FTL) Detect physical fragmentation Remap LBAs of logical fragments Disperse physical fragments Deliver modified LBAs Use new custom interface Update FTL s mapping table Maintain log for reverse mapping Improves I/O performance of mobile storage while minimizing lifetime degradation 13

14 Logical Defragmenter (JanusdL) 1. User triggers e4defrag 2. Detect logical fragments Logical Block Address Space e4defrag Logical Defragmenter Modified LBAs LBA 8 LBA 3 LBA 9 LBA 4 3. Logical defragmentation without data copies 4. Deliver modified LBAs File A Logical Block Address Space File A File A 14

15 Mobile Storage L2P Mapping Table NAND Flash Memory 6. Maintain remapping history Logical Defragmenter (JanusdL) Logical Defragmenter Defrag log L2P Mapping Table 0 NAND PAGE A 1 NAND PAGE B 2 NAND PAGE C Remapping NAND PAGE D NAND PAGE E A B C D L2P Mapping Table E 9 NAND PAGE A NAND PAGE B NAND PAGE C NAND PAGE D NAND PAGE E NAND Page space OOB area (LBA) 15

16 Evaluation Scenarios We collected six different application usage traces Application launching scenarios Interactive application usage scenario (10 minutes) Scenario Chrome Messenger Gmail Facebook Twitter Game Scenario Description Launching app Viewing webpages Launching app Viewing chat records Launching app Viewing s Launching app Viewing online news Launching app Viewing online news Launching Pokemon Go Playing game 16

17 Experimental Settings Application Launch/ Usage Trace Replayer Android smartphone Trace Collector System call trace Emulation at Host-level FTL + Customized SSD 17

18 Result 1: Application Launching Time Application Launching Time (sec) Degree of Logical Fragmentation % -22% -18% -33% -28% baseline e4defrag janusdl janusd 1. The more file fragmentation, the greater the performance improvement 2. Janusd achieves better performance than conventional defragmenter (e4defrag) -53% 18

19 Normalized I/O throughput Result 2: I/O Throughput Baseline e4defrag (4 weeks) e4defrag (2 weeks) e4defrag (1 week) JanusdL (1 week) Janusd (1 week) 10 minute usage scenario Time (Week) 1. Performance degradation occurs even when we defrag smartphone every 2 weeks 2. Conventional defragmenter has limitations in solving physical fragmentation 19

20 Conclusion We have presented a decoupled defragmenter for improving the file system performance JanusdL defrags logical fragmentation without data copies by remapping LBAs with FTL s mapping table JanusdP defrags physical fragmentation by improving I/O parallelism of files Improved application launching times by 32% on average Reduced the amount of data copies by 99.99% on average Future expends Free space defragmentation tool Defrag-on-write() which triggers JanusdL right before write() 20

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