The Five-Minute Rule 20 Years Later (And How Flash Memory Changes The Rules)?

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The Five-Minute Rule20 Years Later(And How
Flash Memory Changes The Rules)?
Goetz Graefe Presented By Abhinav Parate
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Storage Hierarchy
FLASH
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Comparing Flash with Disks
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When should we increase main memory?

• Metrics to decide-
• Cost of infrastructure
• Cost of maintenance
• Mean Time to Failure
• Performance improvement
• Simplest answer Increase RAM size if it is
  insufficient to hold frequently accessed data
  item
• What time period is frequent?

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Cost of accessing a data item

• A disc provides N accesses per second and costs
  D.
• DA D/N Cost of disc access per second
• M Cost of 1 byte of main memory
• I Expected interval when the same data is
  accessed again (in seconds)?
• B Size of data in bytes

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Cost of accessing a data item

• Number of accesses per second for data item 1/I
• Cost if item is accessed from disc DA/I
• Cost if item is available in memory M B
• Keep data item in memory if main memory cost is
  less than disc access cost
• M B lt DA/ I
• I lt DA/ (M B)?
• For 1 KB data item, I lt 400s 5 minutes at 1987
  costs

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The Five-Minute Rule

• In 1987, Keep a 1KB data item in main memory, if
  it is accessed repeatedly in less than 5 minutes.
  
• In 1967, the frequent period was 0.5 s
• In 2007, the authors predicted 5 hour rule
• At actual 2007 prices, the period turned out to
  be little under 6 hours.

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Sample Case

• A database consists of 500,000 records of 1000
  bytes each.
• Peak load consists of 600 transactions per sec.
• Only 6 of data gets 96 accesses and gets
  accessed in lt5min.
• 6 data resides in main memory.
• Remaining data gets accessed via two hard disks
  to support 1 second access time.
• The design saved 3.5m at 1987 costs when
  compared with entirely main-memory design

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Back to Present

• Technology changed
• Multiple cores
• Virtualization
• Size of data increased tremendously
• Gap between RAM and disks performance increased
• FLASH memory comes into the picture!

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Flash memory characteristics

• Purchase cost
• Access Latency
• Bandwidth
• Density
• Power consumption
• Cooling costs
• Everything lies in between RAM and rotating hard
  disks!

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Comparison Flash and Disks
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Desirability of Flash Memory

• Disk I/O is increasingly becoming bottleneck as
  the number of CPU instructions possible in a disk
  I/O is steadily increasing
• A faster intermediate memory in storage hierarchy
  is highly desirable

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Limitation of Flash Memory

• Write-bandwidth is lower than read-bandwidth.
• Re-writing a block requires erasing of entire
  block.
• Reliability 100,000-1M erase and write cycles
• Requires wear levelling mechanism
• Requires agent to erase blocks as soon as they
  are written to hard disk.

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The presentation ahead ...

• Key challenges in using flash memory
• Addressing challenges
• Lots of open questions
• Implications in greening the computing
  infrastructure.

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1 Which hardware interface to use?

• Use DIMM?
• Use Serial-ATA?
• Use new hardware interface?
• Defining and developing new hardware interface is
  time-consuming exercise
• Use one of the existing interfaces

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2 Use as Buffer or Persistent Storage?

• Database systems are concerned with providing
  consistency.
• Databases have large number of small updates and
  must maintain recovery logs.
• Write logs to persistent storage quickly.
• Use Flash as Persistent Storage!

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2 Use as Buffer or Persistent Storage?

• File-systems manipulates the file contents in
  memory and write file to disk in its entirety
• Consistency is achieved via careful write
  ordering, quick write-back and expensive
  file-system checks.
• Page movement between flash and disks is
  expensive if flash is considered as persistent
  storage.
• Use Flash memory as buffer pool!.

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3 How to track Frequent Pages?

• The estimation and administration of frequent
  pages in current system is done through LRU
• Maintain two LRU chains in RAM

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Least Recently Used Chain

• LRU for RAM
• LRU for flash memory

T(N)?
T(N-1)?
T(1)?
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4 How to decide size of RAM and Flash?

• Use Five-Minute Rule!

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5 How to move pages among layers in hierarchy?

• RAM and flash
• DMA Transfer
• Flash and Disk
• DMA (hardware)?
• Transfer buffer in RAM (software)?

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6 How to track Page Locations?

• File systems
• Maintain pointer pages
• Pointer points to data page or run of contiguous
  data pages
• Individual page movement may require breaking up
  run and updating pointer pages

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6 How to track Page Locations?

• Database systems
• Use B-Tree indexes
• Other kinds of indexes have been implemented on
  B-Trees efficiently
• Page movement requires updating pointers in
  parent node and neighbors

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Benefits to Database Systems

• Check Point Processing
• provides consistency in databases
• writes dirty pages to persistent storage
• persistent flash storage is faster
• need to write to disk only if page-replacement
  policy requires
• Recovery Logs
• quick writes

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Benefits to Database Systems

• Query Processing
• Index based selection is faster
• Need to consider index based query plans
• Index joins and intersections
• Example
• Table Scan 100M rows 100s
• Index fetches 10K rows in 100s
• Table Scan is efficient if result has more than
  10K rows.
• Flash index scan fetches 500K rows!

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Problem of Optimal B-tree Page Size

• Two different optimal page sizes

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Implications for Green Computing

• This work's focus is infrastructure cost.
• Energy optimization may lead to different optimal
  page sizes for B-trees.
• Infrastructure cost optimization can lead to
  significant reduction in RAM size and hence,
  lower energy consumption.
• Introduces large flash memory in the system.

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Implications for Green Computing

• P_flash be power consumption with flash memory
• P_noflash be power consumption without flash
• Let T_flash,T_noflash denote system throughput
  with/without flash
• System is green if
• P_flash / P_noflash lt 1
• T_flash / T_noflash gt 1

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Implications for Green Computing

• What if P_flash / P_noflash gt 1?
• In this case, system is green if
• T_flash / T_noflash gt P_flash / P_noflash
• Gain in throughput is higher than extra power
  spent

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Some calculations

• Assume linear relation between number of
  frequently accessed pages and the frequent period
• If M is RAM used in no-flash system
• M/15 is RAM in flash-based system
• 4M is flash memory
• P_flash M/15 x pram 4M x pflash
• P_noflash M x pram
• P_flash lt P_noflash if pflashlt 14/60 pram
• The relationship holds true.

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Conclusions

• Desirable to have faster intermediate memory in
  storage hierarchy.
• Database systems are likely to benefit a lot.
• Things are not clear about file-systems.
• Flash can improve system throughput and reduce
  power consumption.
• Reduction in RAM usage can lead to significant
  power savings.

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Thank You!