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UC Santa Cruz Providing High Reliability in a Minimum Redundancy Archival Storage System Deepavali Bhagwat Kristal Pollack Darrell D. E. Long Ethan L. Miller Storage Systems Research Center University of California, Santa Cruz Thomas Schwarz Computer Engineering Department Santa Clara University Jehan-François Pâris Department of Computer Science University of Houston, Texas

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UC Santa Cruz

Providing High Reliability in a Minimum Redundancy Archival

Storage System

Deepavali BhagwatKristal Pollack

Darrell D. E. LongEthan L. Miller

Storage Systems Research Center

University of California, Santa Cruz

Thomas SchwarzComputer Engineering

DepartmentSanta Clara University

Jehan-François PârisDepartment of Computer

ScienceUniversity of Houston,

Texas

2

Introduction

Archival data will increase ten-fold from 2007 to 2010• J. McKnight, T. Asaro, and B. Babineau, Digital Archiving:End-User Survey and

Market Forecast 2006 - 2010. The Enterprise Strategy Group, Jan. 2006.

Data compression techniques used to reduce storage costs

Deep Store - An archival storage system• Uses interfile and intrafile compression techniques• Uses chunking

Compression hurts reliability• Loss of a shared chunk Disproportionate data loss

Our solution: Reinvest the saved storage space to improve reliability• Selective replication of chunks

Our results:• Better reliability compared to that of mirrored Lempel-Ziv compressed files using only about half of the storage space

3

Deep Store: An overview

Whole File Hashing• Content Addressable Storage

Delta Compression Chunk-based Compression

• File broken down into variable-length chunks using a sliding window technique

• A chunk identifier/digest used to look for identical chunks

• Only unique chunks stored

wfixed size window

chunk end/start

variablechunk size

window fingerprint chunk ID(content address)

sliding window

4

Effects of Compression on Reliability

Chunk-based compression Interfile dependencies

Loss of a shared chunk Disproportionate amount of data loss

Files

Chunks

5

Effects of Compression on Reliability….. Simple experiment to show the effects of interfile

dependencies:• 9.8 GB of data from several websites, The Internet

Archive• Compressed using chunking to 1.83 GB. (5.62 GB using

gzip)• Chunks were mirrored and distributed evenly onto 179

devices, 20 MB each.

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Compression and Reliability

Chunking:• Minimizes redundancies. Gives us excellent compression ratios

• Introduces interfile dependencies• Interfile dependencies are detrimental to reliability

Hence, reintroduce redundancies• Selective replication of chunks

Some chunks more important than others. How important?• The amount of data depending on a chunk (byte count)

• The number of files depending on a chunk (reference count)

Selective replication strategy• Weight of a chunk (w) Number of replicas for a chunk (k)

• We use a heuristic function to calculate k

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k : Number of replicas w : Weight of a chunk a : Base level of replication, independent of w b : To boost the number of replicas for chunks

with high weight

Every chunk is mirrored kmax : Maximum number of replicas

• As replicas increase the gain in reliability obtained as a result of every additional replica reduces

k rounded off to the nearest integer.

Heuristic Function

8

Distribution of Chunks

An archival system receives files in batches Files stored onto a disk until the disk is full

For every file• Chunks extracted and compressed• Unique chunks stored

A non unique chunk stored only if:• The present disk does not contain the chunk• For this chunk, k < kmax

At the end of the batch• All chunks revisited and replicas made for appropriate chunks

A chunk is not proactively replicated • Wait for a chunk’s replica to arrive as a chunk of a future file

• Reduce inter-device dependencies for a file.

9

Experimental Setup

We measure Robustness:• The fraction of the data available given a certain percentage of unavailable storage devices

We use Replication to introduce redundancies• Future work will investigate erasure codes

Data Set:• HTML, PDF, image files from The Internet Archive. (9.8 GB)

• HTML, image (JPG and TIFF), PDF, Microsoft Word files from The Santa Cruz Sentinel (40.22 GB)

We compare the Robustness and Storage Space utilization of archives that use:• Chunking with selective redundancies and• Lempel-Ziv compression with mirroring

10

Details of the Experimental Data

11

When using dependent data (byte count) as a heuristic:• w = D/d• D : sum of the sizes of all files depending on a chunk

• d : average size of a chunk When using the number of files (reference count) as a heuristic:• w = F• F : number of files depending on a chunk

Weight of a Chunk

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Robustness, Effect of varying a, w=F, b=1, kmax=4, The Internet Archive

13

Robustness, Effect of varying a, w=D/d,b=0.4, kmax=4, The Internet Archive

14

Robustness, Effect of limiting k, w=D/d, b=0.55, a=0, The Internet Archive

15

Robustness, Effect of varying b, w=D/d, a=0, kmax=4, The Internet Archive

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Robustness, Effect of varying b, w=D/d, a=0, kmax=4, The Sentinel

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Choice of a Heuristic

Choice of a heuristic depends on the corpus• If file size is indicative of file importance, choose w=D/d

• If file’s importance is independent of its size, choose w=F

• Use the same metric to measure robustness

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Future Work

Study reliability of Deep Store• With a recovery model in place• When using delta compression

Use different redundancy mechanisms such as erasure codes

Data placement in conjunction with hardware statistics

19

Related Work

Many archival systems use Content Addressable Storage:• EMC’s Centera• Variable-length chunks: LBFS• Fixed-size chunks: Venti

OceanStore aims to provide continuous access to persistent data • uses automatic replication for high reliability• erasure codes for high availability

FARSITE: a distributed file system• Replication of metadata, replication chosen avoid the overhead of data reconstruction when using erasure codes

PASIS, Glacier use aggressive replication as a protection against data loss

LOCKSS provides long term access to digital data• uses peer-to-peer audit and repair protocol to preserve the integrity and long-term access to document collections

20

Conclusion

Chunking gives excellent compression ratios but introduces interfile dependencies that adversely affect system reliability.

Selective replication of chunks using heuristics gives• better robustness than mirrored LZ-compressed files

• significantly high storage space efficiency -- only uses about half of the space used by mirrored LZ-compressed files

We use simple replication. Our results will only improve with other forms of redundancies.

UC Santa Cruz

Providing High Reliability in a Minimum Redundancy Archival

Storage System

Deepavali BhagwatKristal Pollack

Darrell D. E. LongEthan L. Miller

Storage Systems Research Center

University of California, Santa Cruz

Thomas SchwarzComputer Engineering

DepartmentSanta Clara University

Jehan-François PârisDepartment of Computer

ScienceUniversity of Houston,

Texas