Logging in Flash-based Database Systems

Logging in Flash-based Database Systems

Lu Zeping2009-06-27

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Outline

• Characteristics of flash memory• Log records in different kinds of Flash devices – In Flash chips - IPL– In SSD– In USB – FlashLogging

• Future work– Recovery processing– Experiments of PTLog

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Outline



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Characteristics of flash memory

• No in-place update

× Out – place update

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• No mechanical latency

MediaRandom-to-Sequential Ratio

Read workload Write workload

Magnetic Disk 4.3-12.3 4.5-10.0

Flash 1.1-1.2 2.4-14.2

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• Asymmetric speed of read/write

Media Read Write Erase

Magnetic disk

12.7ms(2KB)

13.7ms(2KB)

N/A

Flash 80μs(2KB)

200μs(2KB)

1.5ms(128KB)

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The flash memory organization

Device : 1G

Block : 1024

Page : 64

2K Bytes 64 Bytes

Page Register

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The flash memory organization

Device : 1G

Block : 1024

Page : 64

2K Bytes 64 Bytes

Page Register

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Outline



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Design of IPL

• IPL ： In-Page Logging

S. W. Lee, and B. Moon. Design of Flash-Based DBMS: An In-Page Logging Approach. SIGMOD 2007

DatabaseBuffer

in-memorydata page(8KB)

in-memorylog sector (512B)

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Design of IPL


Flash Memory

log area (8KB): 16 sectors

Erase unit (Block): 128KB

15 data pages (8KB each)

….….….

……

……

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IPL Write


DatabaseBuffer

in-memorydata page(8KB) in-memory

log sector (512B)

Flash Memory




….….….

……

……

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IPL Read


DatabaseBuffer


log sector (512B)

Flash Memory




….….….

……

……

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IPL Read


DatabaseBuffer


log sector (512B)

Flash Memory




….….….

……

……

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IPL Merge


Flash Memory

….….….

….….….

……

……

……

……

….

Merge

….

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IPL Merge


Flash Memory

….….….

….….….

……

……

……

……

….

Merge

….

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Outline



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SSD

• SSD = Flash memory + FTL （ Flash Translation Layer ）

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To be used in……

• Accessing data in secondary storage is the main source of bottlenecks in database processing.

• Database table, index, log, temporary table and rollback segment could be created on physically separate storage devices .(SSD+ HDD)

• Log records are always appended to the end of log.

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TPC-B benchmark performance

B. Moon, C. Park, S. W. Lee. A Case for Flash Memory SSD in Enterprise Database Applications. SIGMOD 2008.

We used TPC-B to stress-test the log storage part of the commercial database server.

Due to the group commit mechanism, the average size of a single log write was slightly different between the two storage media.

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TPC-B benchmark performance

B. Moon, C. Park, S. W. Lee. A Case for Flash Memory SSD in Enterprise Database Applications. SIGMOD 2008.

In the case of SSD, the bottleneck in transaction throughput is the processing speed of CPU , while it was not in the case of disk.

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Outline



USB flash memory advantages

• USB ports are widely available.

• USB bus bandwidth is good for logging.

• The hot-plug capability allows easy replacement of drives.

• The price of an individual drive is low.Shimin Chen. FlashLogging: Exploiting Flash Devices for Synchronous Logging Performance. SIGMOD'09. 24

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Flash devices and disk performance

Shimin Chen. FlashLogging: Exploiting Flash Devices for Synchronous Logging Performance. SIGMOD'09.

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Four key challenges

• Efficiently exploiting an array of flash drives.

• Coping with large variance of write latencies.

• Efficient recovery processing.

• Combining USB flash drives with disks for better logging and recovery performance.


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Exploiting an array of flash drivesWhat’s wrong with the conventional array

organization?


In-memory log buffer

……Devices ： N

the stripe unit size is S

Data at address A are found on the k-th device, where k = A/S mod N.⌊ ⌋

Unconventional array design

Shimin Chen. FlashLogging: Exploiting Flash Devices for Synchronous Logging Performance. SIGMOD'09.28


Request queue

Worker Worker Worker……

LSN ： Log Sequence Number

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Coping with outlier Writes


• Sequential writes have large variance

•Outlier prediction is difficult.

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Outlier Detection and Hiding

Worker


Worker

Measure the average sequential write latencies for different request sizes for a device.

Longer than twice the average latency?

Reissue the same request to a different ready device.

Non-decreasing LSN?

Outlier hiding area

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Efficient recovery processing


Two main steps of recovery processing(i) Locating the log record corresponding to the LSN in the last checkpoint.

(ii) Scanning the log sequentially until the crash point.

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Flash devices and disk performance


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FlashLogging design


Request queue

Worker Worker Worker……


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Outline



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Several recovery technologies Log Group

commitPTLog Shadow

pageTxFlash

Update Pattern

In-Place In-Place Out-Place Out-Place Out-Place

Concurrency Granularity

Record Record Page Segment/file

Page

Atomicity Type

Write atomicity

Write atomicity

Propagate atomicity

Propagate atomicity

Propagate atomicity

Commit Overhead

High Low Low High Low

Rollback Overhead

High High None None High

Logging I/O Type

Synchronous Synchronous Synchronous None None

GC overhead

None None Low Low High

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Outline



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Experiments of PTLog

• Berkeley DB

• Linux Nandsim nand_base.c 、 nand_bbt.c 、 nand_ids.c 、 nand_ecc.c 、 nandsim.c 、 diskonchip.c 、 cafe_nand.c 、 s3c2410.c

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Thank you ！Q&A ？

Documents

Logging in Flash-based Database Systems