Freewrite: Creating (Almost) Zero-Cost Writes to SSD

Transcription

1 Freewrite: Creating (Almost) Zero-Cost Writes to SSD Chunyi Liu, Fan Ni, Xingbo Wu, and Song Jiang Xiao Zhang University of Texas at Arlington, USA Northwestern Polytechnical University, China

2 The Achilles' Heel in SSD: Writes SSD has high performance that is not sensitive to access sequentiality However, write operations are expensive and destructive. Write latency can be higher than reads by many times. Out-of-place writes may require mapping change, block erasure, and even page migrations. Expensive garbage collections can be triggered. Writes reduce its limited lifespan. The goal: Remove Unnecessary Writes to the SSD -2-

3 Applications Powerpoint Video Editing Database Key-value Store The Opportunity and Challenge UPDATE FILE (insert/delete/overwrite) READ FILE The OS Kernel Block-level Dedup WRITE UPDATED FILE SSD -3-

4 Applications Powerpoint Video Editing Database Key-value Store The Opportunity and Challenge UPDATE FILE (insert/delete/overwrite) READ FILE The OS Kernel Block-level Dedup SSD? WRITE UPDATED FILE -4-

5 Applications Powerpoint Video Editing Database Key-value Store The Opportunities and Challenge UPDATE FILE (insert/delete/overwrite) Block Boundaries READ FILE The OS Kernel Block-level Dedup SSD -5-

6 Applications Powerpoint Video Editing Database Key-value Store The Opportunities and Challenge UPDATE FILE (insert/delete/overwrite) Block Boundaries READ FILE The OS Kernel Block-level Dedup SSD Duplicated blocks not recognized due to mis-alignment! WRITE UPDATED FILE -6-

7 Freewrite: Retain Alignments to Maximize Duplicates The approach: allow small unused spaces (bubbles) in a file to keep un-updated blocks aligned. Update Ops Before Update After Update w/o Freewrite After Update w/ Freewrite Block # Insert Delete Un-updated blocks Mis-aligned! Insert -7- Unupdated blocks Overwrite Un-updated blocks

8 Freewrite: Retain Alignments to Maximize Duplicates The approach: allow small unused spaces (bubbles) in a file to keep un-updated blocks aligned. Update Ops Before Update After Update w/o Frewrite After Update w/ Freewrite Block # Insert Delete Un-update blocks Mis-aligned! Insert Aligned! -8- Unupdate blocks Overwrite Unupdate blocks Insert a bubble

9 Use Case I: Compaction in LevelDB LevelDB is a key-value store in which KV items are incrementally sorted in a multi-level structure. Level k+1 is much larger than Level k. Adjacent levels are constantly merge-sorted. Freewrite inserts bubbles into the new Level k+1 to reduce its writes. Level k Update Insert bubbles (fake KV items) to retain alignment evel k+1 Merge Sort -9-

10 Use Case II: Garbage Collection in FAWN FAWN is a key-value store always appending KV items in a log file. GC is periodically performed in the log file to remove invalidated items. Freewrite inserts bubbles into the new log file to reduce its writes. Old Log Garbage Collection New Log Insert bubbles (fake KV items) to retain alignment -10-

11 Experimental Setup Implemented at the application level (LevelDB and FAWN) Use Dmdedup as block-level dedup. Use Samsung 840 EVO 1GB SSD. Use put requests with 4% repeated keys. Threshold of bubble size is 12.5% of block size (512B), and total bubble space overhead is capped at 5%. Three key distributions: uniform, clustered, and mixed. -11-

12 Experiment Results (LevelDB) 22% improvement of Throughput 24% reduction of written data -12-

13 Experiment Results (FAWN) 4.5X improvement of Throughput 8.2X reduction of written data -13-

14 Summary Freewrite creates deduplication opportunities to leverage the readily available block-level deduplication system. Freewrite improves write performance and helps to extend SSD lifetime. The overhead of using bubbles is very limited. -14-