Outline Motivation and background Read Write Read “seek” time Good sequential read Write Write log block (buffer)‏ “1” !=”0”? Conclusion and future work

Motivation Era for Flash? We only get BLACK BOX. Nice properties: Fast random access; Shock / Temperature resistance ; Smaller size and weight. Energy saving. Reducing price + increasing volume. We only get BLACK BOX. Industry company may not tell? No systematic performance study literature?

Flash Memory Organization Read and Write Each page: 2KB,4KB Each block: 64,128 pages Read and Write Read in pages ~50us Write in pages ~800us Any write needs block erase ~1500us Block 0 Block 1 Page 0 Block n-1 Page 1 Page 2 ...... Page 3 ...... Page m-1 Chip organization

Outline Motivation and background Read Write Read “seek” time. Good sequential read. Write Write buffer size. “1” !=”0”? Conclusion and future work

Read Assumptions and basic knowledge Experiments Setup Uniform random read time? Good sequential read performance? Experiments Setup Fedora Core 7.0,1GHZ CPU, 1G Memory Flash memory (I) Kingston DataTraveler 1G (II) PNY Attaché 2G (III) Unknown 1G

Random Read-- “seek” time

Sequential Read –good!

Sequential Read –Scale up

Read –what we can do? Technology aware FS Block group VS Cylinder group To Group files. Random read is good but not perfect. To decrement random accesses.

Outline Motivation and background Read Write Conclusion Read “seek” time Good sequential read Write Write log block (buffer)‏ “1” !=”0”? Conclusion

Write Assumptions and basic knowledge Bad random write performance Needs block erase (1 page--> block erase)‏ Good sequential write performance Limited block erase times (64 pages--> block erase) Reason : Log (buffer) and Merge

Write -- merge 1.Write valid valid valid 2.merge 2.merge valid valid Log Block Pool Data Block Free Data Block Log Block 3.erase 3.erase

Random Write -- bad!

Continuous write – great relief from erase

Write -- log block What is it? Why is it used? Flash block as write buffer Correspond to one flash block at one time Why is it used? Hard disk : clustering writes; save redundant Flash disk: reduce erase times Interesting: Log block size and usage

Log block -- size Motivation Size: 64 pages 128 pages Trade off: Large merge time VS frequent merge Size: 64 pages 128 pages

Determine size of Log block pool Repeat writing one page into set of continuous flash blocks sequentially. Check the time cost.

Log block pool -- Use Strategy Log block pool use FIFO to reclaim used blocks Repeatedly writing less than 16 pages into one flash block does not trigger data merge.

“0” != “1”

How about 50%?

Write -- what we can do? New file system for flash Modified LFS Log block !=write buffer Special policy for frequently updated data, e.g inodes Anticipatory scheduling More flexible. Directly execute any write request in one data block associated with log block. Flip “1” to “0”, “0” to “1” may save time (attributed to Remzi)‏

Conclusion Comprehensive study of the read and write performance of flash memory Design a relatively systematic method to study the flash memory as a black box Find some interesting and potentially useful properties, e.g. “1”!=”0” ; “seek” time Apply similar performance study strategy to SSD and check whether the properties still hold

Q&A

Random Read-- “seek” time

Random Read-- “seek” time