FlashTier: A Lightweight, Consistent and Durable Storage Cache Mohit Saxena, Michael M. Swift and Yiying Zhang (University of Wisconsin-Madison) Proceedings of the 7th ACM european conference on Computer Systems (EuroSys’12) July 25, 2013 발표: Jeong Su Park (jspark@archi.snu.ac.kr)

SSD as a cache Host OS DRAM Cache manager SSD - Cheaper than DRAM - Improve cold start performance (Non-volatile nature) SSD - Faster than HDD HDD

Drawbacks of conventional SSD as a cache HDD SSD SSD address space (LBA-s) Host OS LBA-d to LBA-s Mapping info Cache manager HDD address space (LBA-d) SSD Consumes a lot of DRAM resources Should guarantees consistency & durability of mapping info Garbage collection degrades performance HDD

Solid State Cache (SSC) HDD SSC SSC address space (Same as LBA-d) Host OS LBA-d to LBA-s Mapping info < FlashTier > Cache manager HDD address space (LBA-d) SSC Unified address space -> Remove mapping info (DRAM resource save) Interface expansion -> Garbage collection overhead reduction (Performance enhancement) Consistency & durability guarantee with checkpoint & roll-forward (Fast crash recovery) HDD

Unified address space Cache manager Cache manager SSD SSC HDD HDD < General purpose SSD > LBA-d to LBA-s < Solid State Cache > LBA-d to LBA-s Cache manager Cache manager 1. Look-up 1. Request 2. Request (Hit) 2. Result (Hit/Miss) 3. If missed 2. Request (Miss) SSD SSD address space (LBA-s) SSC Same as LBA-d LBA-s to PA LBA-d to PA (Physical Address) HDD HDD HDD address space (LBA-d) HDD address space (LBA-d)

Interface expansion SSC Cache manager 1. Write-through policy HDD Physical blocks Clean Meta info Invalid LBA-d to PA Block info (Liveness/ Data status) 1. Write-through policy Choose victim (all clean | invalid) and erase only when remaining free blocks are insufficient (Silent-evict) Write-clean HDD Cache manager SSC Clean-data Clean-data Hit ratio Read Hit/miss Read Always hit Clean-data

Interface expansion SSC Cache manager 2. Write-back policy HDD Physical blocks Clean Meta info Invalid LBA-d to PA Dirty Block info (Liveness/ Data status) Erasure is not permitted 2. Write-back policy Choose victim (all clean | invalid) and erase only when remaining free blocks are insufficient (Silent-evict) Write-dirty HDD Cache manager SSC Dirty-data Read Hit/miss Read Always hit

Interface expansion SSC Cache manager 2. Write-back policy HDD Physical blocks Clean Meta info Invalid LBA-d to PA Dirty Block info (Liveness/ Data status) - Clean : SSC mapping info preserved until silent-evict occurs - Evict : SSC mapping info vanishes immediatly 2. Write-back policy Choose victim (all clean | invalid) and erase only when remaining free blocks are insufficient (Silent-evict) Clean or evict HDD Cache manager SSC Dirty block table LBA-d clean evict Dirty-data Dirty-data

Interface expansion / crash recovery Physical blocks (Checkpoint area) Meta info LBA-d to PA Periodically checkpoint entire set with seq. # Block info (Liveness/ Data status) Physical blocks (Logging area) 2. Write-back policy Logging updated portion with seq. # Dirty block table re-construction with ‘exist’ query During crash recovery Exists Cache manager SSC Dirty block table Restore most-recent checkpoint info & roll-forward logging area during crash recovery LBA-d range Dirty data status within queried range

Evaluation Linux kernel 2.6.33 SSD Vs. SSC (fixed page-mapped area) (write intensive) SSD (read intensive) FlashSim Simulator FlashCache Linux kernel 2.6.33 Modified FlashSim Simulator + Custom Hybrid mapping FTL Cache manager (Modified FlashCache @ Facebook) Trace replay Vs. SSC (fixed page-mapped area) SSC SSC-R (flexible page-mapped area) ※ Hardware implementation M. Saxena, Y. Zhang, M. M. Swift, A. C. Arpaci Dusseau, and R. H. Arpaci Dusseau, “Getting Real: Lessons in Transitioning Research Simulations into Hardware Systems,” FAST 13.

Results Write intensive Read intensive Proposed Proposed device cache manager Native : Host should reconstruct entire mapping info. before running => Entire storage area scan needed (mapping info. is stored in spare area) Proposed : Host may defer reconstructing dirty block info. + dirty block info. can be quickly reconstructed from checkpoint & roll-forward. (Not fair….)

Results