Repairing Write Performance on Flash Devices

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Repairing Write Performance on Flash Devices

• Radu Stoica, Manos Athanassoulis, Ryan
  Johnson,
• Anastasia Ailamaki

Ecole Polytechnique Fédérale de
Lausanne Carnegie Mellon
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Tape is Dead, Disk is Tape, Flash is Disk

• Slowly replacing HDDs (price , capacity )
• Fast, reliable, efficient
• Potentially huge impact
• Slow random write
• Read/write asymmetry
• -gt not a HDD drop-in replacement

Jim Gray, CIDR 2007
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DBMS I/O today
Request
DBMS
Data requirements
HDD optimized I/O pattern
Block Device API
Flash optimized I/O pattern
Flash device
Flash memory access
Inadequate device abstraction
Flash devices are not HDD drop-in replacements
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Random Writes Fusion ioDrive

• Microbenchmark 8 kiB random writes

Unpredictability
94 performance drop
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Stabilizing Random Writes

• Change data placement
• Flash friendly I/O pattern
• Avoid all random writes
• Minimal changes to database engine
• 6-9x speedup for OLTP-like access patterns

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Overview

• Random Write how big of a problem?
• Random Write why still a problem?
• Append-Pack Data Placement
• Experimental results

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Related work
Request
DBMS
Data requirements
Flash-opt. DB Algs.
HDD optimized I/O pattern
Data placement
Block Device API
Flash FS
Flash optimized I/O pattern
Flash device
FTL
Flash memory access
No solution for OLTP workloads
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Random Write Other devices
Vendor advertised performance
Rand. Write Rand. Read
Mtron SSD
Rand. Write causes unpredictability
Graph from uFlip, Bouganim et al. CIDR 2009
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Random Writes Fusion ioDrive

• Microbenchmark 8 kiB random writes

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Sequential Writes Fusion ioDrive

• Microbenchmark 128kiB sequential write

Seq. Writing Good Stable Performance
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Idea Change Data Placement

• Flash friendly I/O pattern
• Avoid all Random Writes
• Write in big chunks
• Tradeoffs additional work
• Give up seq. reads (SR and RR similar
  performance)
• More seq. writing
• Other overheads

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Overview

• Random Write how big of a problem?
• Random Write why still a problem?
• Append-Pack Data Placement
• Theoretical model
• Experimental results

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Append-Pack Algorithm
Update page
Update page
Update page
No more space
Write hot dataset
Write seq.
Reclaim space
No in-place updates
Filter cold pages
Write cold dataset
Reclaim space
Log start
Valid page
Log end
Invalid page
How much additional work?
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Theoretical Page Reclaiming Overhead

• Update pages uniformly
• Equal prob. to replace a page
• valid pages?

prob(valid) f (a) ? e -a
a
Worst case 36 Easily achievable 6-11
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Theoretical Speedup

• Traditional Random Write I/O latency TRW
• New latency TSWprob(valid)(TRR TSW)
• Conservative assumption TRW 10TSW

a sizeof(device) / sizeof(data)
Up to 7x speedup
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Overview

• RW how big of a problem?
• RW why still a problem?
• Append-Pack Data Layout
• Experimental results

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Experimental setup

• 4x Quad-core Opteron
• X86_64-linux v2.6.18
• Fusion ioDrive 160GB PCIe
• 8 kiB I/Os, Direct I/O
• Parallel threads 16
• Firmware runs on host
• Append-Pack implemented as shim library

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OLTP microbenchmark

• Microbenchmark 50 Rand Write / 50 Rand Read

FTL?
9x improvement
Time (s)
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OLTP Microbenchmark Overview
Performance better than predicted
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What to remember

• Flash ? HDD
• We leverage
• Sequential Writing to avoid Random Writing
• Random Reading as good as Sequential Reading
• Append-pack eliminate Random Writes
• 6-9x speedup

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• Thank you!
• http//dias.epfl.ch

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Backup
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FTLs

• Fully-associative sector translation Lee et al.
  07
• Superblock FTL Kang et el. 06
• Locality-Aware Sector Translation Lee et al.
  08
• No solution for all workloads
• Static tradeoffs workload independence
• Lack of semantic knowledge
• Wrong I/O patterns
• -gt complicated software layers destroy
  predictability

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Flash FS

• Flash aware file systems
• JFFS2
• YAFFS
• No FTL required (handle wear leveling)
• Mostly for embedded devices

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Other Flash Devices - Backup
Device RR (IOPS) RW (IOPS) SW (MB/s) SR (MB/s)
Intel x25-E 35,000 3,300 170 250
Memoright GT 10,000 500 130 120
Solidware 10,000 1,000 110 110
Fusion ioDrive 116,046 93,199 (75/25 mix) 750 670
Vendor advertised performance
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Experimental Results - Backup
RR/RW Baseline Append/Pack Speedup Prediction
50/50 38 MiB/s 349 MiB/s 9.1 6.2
75/25 48 MiB/s 397 MiB/s 8.3 4.3
90/10 131 MiB/s 541 MiB/s 4.1 2.5
(a 2 in all experiments)
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OLTP microbenchmark - Backup
50 RW/50 RR - before

• 50 RW/50 RR - after

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OLTP Microbenchmark - Backup
Traditional I/O
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OLTP Microbenchmark - Backup
Append-Pack