Wolf a novel reordering write buffer to boost the performance of log structured file systems
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WOLF – A Novel Reordering Write Buffer to Boost the Performance of Log-Structured File Systems. Jun Wang, Yiming Hu Proceedings of the FAST 2002 Conference on File and Storage Technologies Jan, 28-30, 2002. Log Structured File System.

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Wolf a novel reordering write buffer to boost the performance of log structured file systems l.jpg

WOLF– A Novel Reordering Write Buffer to Boost the Performance of Log-Structured File Systems

Jun Wang, Yiming Hu

Proceedings of the FAST 2002 Conference on File and Storage Technologies

Jan, 28-30, 2002


Log structured file system l.jpg
Log Structured File System

  • Writes all modifications to disk sequentially in a log-like structure

    • Speeding up both file writing and crash recovery

  • Log is the only structure on disk

  • Segment cleaner (garbage collector) needed


Problems of lfs 1 2 l.jpg
Problems of LFS (1/2)

  • Garbage collection overhead(a.k.a. segment cleaning overhead)

    • Especially under random updates and accesses

  • Several schemes have been proposed to speed up garbage collection process


Problems of lfs 2 2 l.jpg
Problems of LFS (2/2)

  • Proposed schemes

    • Greedy/benefit-to-cost cleaning

    • Hole-plugging policy

    • Adaptive algorithm

    • And more…

  • All concentrated to make cleaner efficiently after files are written to disk


New scheme 1 2 l.jpg
New Scheme (1/2)

  • WOLF

    • reordering Write buffer Of Log-structured File system

    • Uses two or more segment buffers

      • When write data arrives, system sorts them into different buffers according to their expected longevity

      • One for active data, another for less-active data, ……


New scheme 2 2 l.jpg
New Scheme (2/2)

  • WOLF (cont’d)

    • Leads to “bimodal distribution”

      • Segments are either ‘mostly full’ or‘mostly empty’

      • Cleaner can select many nearly empty segments to clean and compact into a small number of segments

    • By re-organizing data in RAM before they reach the disk, this scheme also make the system do less garbage collection work


Lfs writing process 1 l.jpg
LFS writing process – 1

Three kinds of blocks

Empty block

Valid data block

Invalidated block

Data blocks first enter a segment buffer

Data

Buffer

Note : This slide and following 3 slides are copied from

author(Jun Wang)’s own FAST 2002 presentation slides.


Lfs writing process 2 l.jpg
LFS writing process – 2

Buffer will be written to the disk when full

……

Disk

After a while, many blocks in segments are invalidated,

Leaving holes and require garbage collection

……

Disk


Wolf writing process 1 l.jpg
WOLF writing process – 1

Four kinds of blocks

Valid active

block

Valid inactive block

Empty

block

Invalidated block

Data blocks first enter one of two segment buffers

based on expected activities

Data

Buffer1

Buffer2


Wolf writing process 2 l.jpg
WOLF writing process – 2

Buffer will be written to the disk when full

……

Disk

After a while, most blocks in active segments are

invalidated,While most in the inactive segments are intact.

……

Disk


Wolf design write 1 2 l.jpg
WOLF Design – Write (1/2)

  • Key problem

    • How to separate active data from inactive data and put them into different buffers accordingly


Wolf design write 2 2 l.jpg
WOLF Design – Write (2/2)

  • Heuristic adaptive grouping algorithm

    • Each block in reorder buffer has a reference count (initially zero)

    • Reference count is incremented when the block is accessed

    • Count is reset to zero after certain period of time (timebar(initially 10min))

    • Timebar could be adaptively tuned for various incoming accesses (doubled, or halved)


Wolf design read l.jpg
WOLF Design – Read

  • WOLF only changes the write cache structure of LFS

    • Read operation are not affected

  • WOLF read performance is similar to that of LFS

    • But, when system is heavily loaded, WOLF should have better read performance because of its more efficient garbage collection process


Wolf design gc l.jpg
WOLF Design – GC

  • use adaptive approach (Matthews SOSP97)

  • Select either“benefit-to-cost cleanning” or“hole-plugging”depending on“cost-benefit estimates”


Experiment environment 1 2 l.jpg
Experiment Environment(1/2)

  • WOLF simulator

    • LFS simulator (port from Sprite LFS)

    • Disk simulator


Experiment environment 2 2 l.jpg
Experiment Environment(2/2)

  • Workload model

    • Four real world traces

      • INS, RES, SITAR, HARP

    • Four synthetic traces

      • Uniform

      • Hot-and-cold

      • Ephemeral small file

      • Transaction processing suite (TPC-D)


Performance result 1 2 l.jpg
Performance Result (1/2)

Note : This slide and following slide are copied from

author(Jun Wang)’s own FAST 2002 presentation slides.



T h e e n d l.jpg
T h e E n d

Any question?