1. 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

2. 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

3. 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

4. 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

5. 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)

6. 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

7. 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

8. 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

9. 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

10. Evaluation Linux kernel 2.6.33 SSD Vs. SSC (fixed page-mapped area)
(write intensive)
SSD
(read intensive)
FlashSim
Simulator
FlashCache
Linux kernel
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.

11. 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….)

12. Results