Leverage Similarity and Locality to Enhance
Fingerprint Prefetching of Data Deduplication
Yongtao Zhou, Yuhui Deng, Junjie Xie
Department of Computer Science, Jinan University, Guangzhou, 510632,
P. R.China
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Agenda
• Introduction
• Related work
• Motivation
• System overview
• Evaluation
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Introduction
• IDC: 95% redundant data in the backup systems; 75% redundant data across the
digital world
 Consumes IT resources and expensive network bandwidth
• Data deduplication: eliminate redundant data by storing only one data copy
Fingerprint
Chunk
algorithm
Files
Fingerprints to large too
store all fingerprints in
memory
MD5
Data blocks
Hash Table
B+ Tree
A
B
G
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Introduction
• Querying fingerprints incurs disk bottleneck
 The size of fingerprints are too large too be cached in memory
 Cache hit ration very low (lack temporal locality)
 The IOPS of disk drivers is limited
800TB unique data
MD5 signature Avg.8KB
Chunk
A large portion of fingerprints have to be stored on disk drives
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Locality based strategies: DDFS
• Locality: segments tend to reappear in the same or very similar sequences with other
segments. This is because most data from previous backup has a slight modifications.
RAM
Prefetching!!!!
Index
A B C A
...
Bloom Filter
...
A E I A
ABC
DE F
H I J
Poor deduplication performance
when there is little or no locality in
datasets
?
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Similarity based method: Extreme Binning
RAM
DISK
Similarity Index
C1 C2 Cn
Bin(C1)
Files
Bin(C2)
Bin(Cn)
Fail to identify and thus remove
significant amounts of redundant data
when there is a lack of similarity among
files
It uses a two-leve lindex structure made up of similarity characteristic value and the granularity of bin.
Extreme Bining stores the similarity characteristics value in RAM.
Extreme Bining only identifies the redundant data in the same bin, even though neighbouring bins
may have identical data blocks.
This results in some redundant data blocks so as to degrade the deduplication ratio.
the deduplication ration of Extreme Bining heavily relies on the similarity degree of data streams.
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SSD based approach
• Fingerprint lookup disk bottleneck
 The IOPS of disk drive is limited
HDD VS SSD
• Some studies alleviate disk bottleneck by using SSD
 Dedupv1, ChunkStash
• SDD is still very expensive in contrast to disk drives.
• The performance of random and small writes becomes a new bottleneck of SSD
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Our approach
• A fingerprint prefetching approach by using the file similarity to enhance the
deduplication performance
• The locality of fingerprints are maintained by arranging the fingerprints in terms of the
sequence of the backup data stream
• The overhead of different similarity identification algorithms are investigated, and the
impacts of those algorithms on data deduplication are evaluated in contrast to previous
studies Extreme Binning, Silo, FPP
• This approach does not impact the deduplication ration
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System architecture
Implementation in LessFS
Implementation in Tokyo Cabinet
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Storage structure for fingerprints
The locality of fingerprints
are maintained by arranging
the fingerprints in terms of
the sequence of the backup
data stream
Loss the locality of
fingerprints
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The process of fingerprints prefetching
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Evaluation
• Implement a real prototype based on LessFS and Tokyo Cabinet
• Three similarity identification algorithms FPP, PAS and Simhash are implemented in
the Similar File Identification Module
• Ubuntu operation system(Kernel version is 3.5.0-17) ,1GB memory, 2:4GHz Intel(R)
Xeon(R) CPU
• We take four full backups to evaluate the system like what DDFS does.
• Four data sets backup1, backup2, backup3 and backup4 to perform the evaluation
 10GB, 15GB, 20GB and 25GB, and the numbers of files are 3073, 4694, 6539 and 9910,
respectively.
 We choose fixed-size chunk algorithm. The chunk size is 4KB, 8KB, 16KB, 32KB, 64KB and 128KB
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FPP and PAS
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Simhash
• Simhash is a member of the local sensitive hash
• Simhash has the property that the fingerprints of similar files differ in a small number of
bit positions
• Actual runs at Google web search engine
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Data sets
The file size distribution matches the previous studies.
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Deduplication ratio
• We measure the size of unique data blocks by using three different similarity
identification algorithms including FPP, PAS and Simhash with four full backups
• When the chunk size is 4KB, the unique data blocks are 14GB, and the data
deduplication ratios are 3.93 across the three cases.
• The performance is the same as that of the baseline system LessFS.
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Time overhead of fingerprint lookup
• 𝑇𝑠 : the time of similarity detection
• 𝑇𝑝 : the time of fingerprint prefetch
• 𝑇𝑓 : the time of fingerprint lookup
• The overall overhead of fingerprint lookup 𝑇𝑡 = 𝑇𝑠 + 𝑇𝑝 + 𝑇𝑓
• For Base has 𝑇𝑠 = 𝑇𝑝 = 0
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Time overhead of fingerprint lookup
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CPU utilization
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Memory utilization
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Conclusion
• Proposes a fingerprint prefetching approach by preserving the locality of fingerprint in
the form of backup data stream as well as taking advantage of file similarity
• The proposed method can effectively alleviate the disk bottleneck with acceptable
overhead of CPU, memory, and storage when performing fingerprint lookup, thus
improving the throughput of data deduplication
• Does not impact the data deduplication ratio
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Reference
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SSD: http://en.wikipedia.org/wiki/Solid-state_drive
HDD vs SSD: http://www.diffen.com/difference/HDD_vs_SSD
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Thank you!
Question?
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