BIGTABLE: A DISTRIBUTED

STORAGE SYSTEM FOR

STRUCTURED DATA

Presenter: Qiping Wei1

Introduction

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Bigtable is a distributed storage system for managing structured data.– designed to scale to petabytes of data

– many projects at Google store data in it• web indexing, Google earth and Google finance,...

• different data sizes: URL, web pages, satellite imagery,...

• different latency requirements: backend bulk processing to real time data serving

– Provide a flexible high performance solution

– Rely on Chubby lock service

– Use Google file system to store data items

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Row key

timestamp

column key

Property: reads of short row ranges are efficient

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Row key value

AA … Value

AZ … Value

BA … Value

BU … Value

CB … Value

CM … Value…

……

……

Row range

tablet

A row rangeUnit of distribution

g

Good locality: obtained by placing access-related data together.

Answer: select row keys in the way that keys can be grouped together.

Eg. two row keys: apple, banana

revised row keys: fruit.apple, fruit.banana

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g

• get a lot of useful data by doing only one read request.

• Reduce the number of read requests.

• Experience high speed of read operations.

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Architecture• Bigtable has three major components:

– One master server• Assigns tablets to tablet servers

• Detects addition and expiration of tablet servers

• Balances the load of tablet servers

• Collect garbage in GFS

• Handles schema changes

– Many tablet servers• Manage a set of tablets

• Handle read/write requests from clients

• Store data in GFS

– A library linked to every client• Communicate tablet servers directly to reads and writes

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8

Tabletserver

Tabletserver

Tabletserver

One Master

Bigtable CientLibrary ……

read/write request

•Balance load•Handle schema changes•Collect garbage

• Assign tablets • Detect addition & expiration of tablet servers

Manage a set of tabletsHandle read/write requestStore data in GFS

Data items: either in log files or in SSTables.

GFS: provide data reliability

By having multiple replicas

Of data.

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SSTablesLog files

GFS

memtable

Mem

immutable: not allowed to modify the data.

This feature has many benefits:

• simplify various parts of Bigtable

eg. cache maintaining is easy;concurrency control implementation is efficient

• split tablet quickly

• Restore data 10

Assume that the KV item exists, start searching from memtable, then SSTables from low level to high level until find it.

Here are the steps:

• Check in-memory index first

• Find the appropriate block

• Check Bloom Filter to see if the KV item is there.

• If yes, read the block from disk and get the value.

• If no, continue the above steps until find the block and get the value.

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Benefits resulting from sorting the key:

• support range search

• Reduce index size

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• these updates: updates committed to the commit log.

• How to do an update?

• based on write operation

• depends on the manner of searching key

from top to bottom

• chooses the latest key 13

• Why exist?

– Limited memtable size

– Immutable SSTables

– Multiple versions allowed

• Why exist in multiple SSTables?

– SSTables from different levels can have overlap ranges

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• Minor compaction: converting the memtable to a new SSTable.

• Major compaction: – turning multiple SSTables to a new large SSTable.

– No deletion information or deleted data

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GFS

Memtable

SSTable

SSTable…

Mem

SSTable

Major compaction

Minor compaction

Contributions from major compaction:

• bound the number of SSTables

• reclaim resources used by deleted data

• Remove overlapped ranges to support range search

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GFS

Read OpMemtable

SSTable

SSTable…

Mem

SSTable

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4. Major CMPTGFS

Delete Op

Memtable

SSTable

SSTable

…

Mem

SSTable

Commit log

3.Minor CMPT

1. write a special deletion record

2. Insert record

Conclusion• Bigtable is a distributed multi-dimensional sorted

map indexed by a row key, a column key and a timestamp.

• The sorted feature has many benefits:– support range search– reduce index size– support read/write operations– allow to manipulate row keys to get good locality

• Bigtable provides high data reliability through GFS.

• The immutability and compaction of SSTAblessimplify and improve the performance of Bigtable.

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Reference

• F. Chang, J. Dean, S. Ghemawat, W.C. Hsieh, D.A. Wallach, M. Burrows, T. Chandra, A. Fikes, R.E. Gruber. Bigtable: A Distributed Storage System for Structured Data. OSDI, 2006

• Lecture: the Google Bigtable. https://www.slideshare.net/romain_jacotin/undestand-google-bigtable-is-as-easy-as-playing-lego-bricks-lecture-by-romain-jacotinhe. October,2014

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