1. Join Processing for Flash SSDs: Remembering Past Lessons
Jaeyoung Do, Jignesh M. Patel
Department of Computer Sciences
University of Wisconsin-Madison
All right. Welcome to my presentation. My name is Jaeyoung, Do. Today I’m going to talk about the performance of join algorithms on flash SSDs.

2. Flash Solid State Drives (SSDs)
▶ Benefits of Flash SSDs
No seek time, small latency
▶ Flash Densities
MB NAND Flash
GB NAND Flash
TB NAND Flash
(predicted by Samsung)
▶ Flash Prices
Decreasing continuously GB
What is flash Solid State Drives. Flash SSDs are data storage devices that use NAND flash memory chips. Currently, the most available and popular storage device is absolutely magnetic hard disk drives. However, since flash SSDs have many benefits compared to magnetic hard disk drives, they are expected to gradually replace hard disks as the primary permanent storage media in large data centers.
These two graphs show the change of flash densities and flash prices as time goes by. As its capacity continues to increase and price drops, Jim Gray’s prediction of “Flash is disk, disk is tape, and tape is dead” is coming close to reality in many applications.
Then, what are advantages of flash SSDs over magnetic hard disks?
$/MB
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3. Flash SSDs for DBMSs still apply to flash SSDs?
▶ Many previous works with flash SSDs
Flash-based DBMS [Gray ACM Queue 08, Graefe DaMoN07]
In-Page Logging [Lee VLDB07]
Transaction Processing [Lee VLDB08]
B+ Tree Index [Li ICDE09]
I/O Benchmarks [Bouganim CIDR09]
▶ We focus on Join algorithms
New join algorithms [Shah DaMoN08, Tsirogiannis SIGMOD09]
So, the use of flash SSDs for DBMSs are being actively studied in various fields including transaction processing and b+ tree index. In this work, we focus on joins which are common, but expensive query processing operations. There are already a few researches in order to develop new join algorithms on flash SSDs. Rather than developing new join algorithm, our aim of this research is to see what lessons join techniques that worked for magnetic hard disk drives continue to work with flash SSDs. So, we wanna see what happen if we just replace magnetic HDDs with flash SSDs, and examine the effects of tuning parameters of join performance.
But we already got plenty of lessons that we have learnt over three decases of desinging and turinng join algorihtms for hard disk drive based systems. If joins were done with flash SSDs, would theses lessons be meaningless? As we will see later, the answer is NO. Obviously many lessons learnt for magneis
HDDs still work for flash SSDs.
The focust of this work
Purpose here is to
Different
XX however,
What lessons learnt for magnetic HDDs
still apply to flash SSDs?
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4. Goals 2. Providing better insights for join performance on flash SSDs
▶Demonstrate the importance of recalling past lessons about efficient joins in magnetic HDDs ▶ Explore the effects of various parameters for joins on flash SSDs
1. Not inventing new join algorithms
2. Providing better insights for join
performance on flash SSDs
Therefore, the first goal of our research is that we want to recall some of the important lessons about efficient join processing in magnetic hard disks determining if these lessons also apply to joins using flash SSDs. In addition, we want to see how tuned parameters based on characteristic of magnetic HDDs works for joins on flash SSDs.
Better staring points when designing new join algori
Wthms for flash SSDs.
We want to see the effects of tuned parameters that are based on characteristics of HDD, and how they work for join on flahs SSDs
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5. Our Approach ▶ Investigate four popular ad hoc join algorithms
Block Nested Loops Join (BNL)
Sort-Merge Join (SM)
Grace Hash Join (GH)
Hybrid Hash Join (HH)
▶ Conduct experiments as varying various parameters
Memory buffer pool size
Page size
I/O unit size
To achieve the goals, we have investigated four ad hoc join algorithms which are popular in both literacture and industry. four popular ad hoc join algorithms, namely BNL, SM, GH, HH. Then, we have conducted some experiments to see the effect of tuned parameters such as …
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6. Roadmap
▶ Introduction / Goals ▶ Ad hoc join algorithms ▶ Experimental Results ▶ Conclusion / Future Work
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7. Assumptions
▶ Blocked I/O is available ▶ We use the buffer allocation strategy tuned for magnetic HDDs [Haas et al. VLDB97]
Before explaining the join algorithms considered in our work, let me give some assumptions. First, Rather than fetching one page in each I/O operation we used blocked I/O to sequentially read and write multiple pages per an I/O operation. In the case of magnetic hard disk drives, blocked I/O is useful because it amortizes the cost of expensive disk seeks and rotational delays.
Also, a lot is known about how to optimize joins with magnetic HDDs to use the available memory buffer pool effectively. Specifically, Hass showed that the right buffer pool allocation strategy can have a huge impact – upto 400% improvements in some cases. In this work, we use the same buffer alolcation method for both flash SSDs and magnetic HDDs. While these allocations may not be optimal for flash SSDs, our goal here is to start with the best allocation strategy for magnetic HDDs, and explore what happens if we simply use the same settings when replacing a magnetic HDD with a flash SSD.
per a disk I/O.
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8. Block Nested Loops Join
IS=
Block Nested Loops Join
Input Buffer for R
Result
Input Buffer for S
Buffer Pool R S
IS=
, IR=B-IS
Disk
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9. Sort-Merge Join Buffer Pool Buffer Pool Result Disk Disk Working Space
Input Buffer
Input Buffer
Result
Input
Buffer
Output Buffer
Input Buffer R
Optimization is out of concerns why?> S
Disk
Disk
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10. Grace Hash Join Buffer Pool Buffer Pool Result Disk Disk Input
Bucket 1 Z
Input
Buffer for R
Result
Input
Buffer for S h
Bucket k R S
Disk
Disk
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11. Hybrid Hash Join Buffer Pool Buffer Pool Result Result Disk Disk In
Memory
Bucket
Bucket 1
Result
Input
Buffer for R
Result
Input
Buffer
Input
Buffer for S h
Bucket k R S
Disk
Disk
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12. Roadmap
▶ Introduction / Goals ▶ Ad-hoc join algorithms ▶ Experimental Results ▶ Conclusion / Future Work
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13. Experimental Setup ▶ A single-thread and light-weight database engine
▶ Flash SSD and magnetic HDD
OCZ Core Series 2.5” SATA 60 GB
TOSHIBA 5400 RPM 320 GB
▶ Data Set: TPC-H
Customer: 730 MB
Orders: 5 GB
▶ Platform
Dual Core 3.2 GHz Intel Pentium, Red Hat
Max. DB buffer pool size: 600 MB
SSD
HDD
None
Avg. Seek Time
12 ms
0.35 ms
Avg. Latency
5.56 ms
120 MB/sec
Read Data
Transfer Rate
34 MB/sec
80 MB/sec
Write Data $
(3.85 $/GB)
Price $
(0.36 $/GB)
MAX BUFER pAGE
Source: OCZ and TOSHIBA
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14. Effect of Blocked I/O (HDD)
500 MB Buffer Pool, 8 KB Page Size
Non-Blocked I/O
Blocked I/O
2.1X
CPU time
I/O time
2.2X
Join Time (sec)
2.0X
2.3X
500 MB, 8 KB
BNL SM GH HH BNL SM GH HH
HDD
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15. Effect of Blocked I/O (SSD)
500 MB Buffer Pool, 8 KB Page Size
Non-Blocked I/O
Blocked I/O
CPU time
I/O time
1.7X
Join Time (sec)
1.6X
1.7X
1.9X
BNL SM GH HH BNL SM GH HH
SSD
Using blocked I/O is critical
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16. Joins are I/O Bound? (HDD)
8 KB Page Size, Blocked I/O
Buffer Pool Size
200 MB
500 MB
CPU time
I/O time
0.68
0.65
0.69
Join Time (sec)
Join Time (sec)
0.62
0.58
0.34
0.61
0.31
8 KB, Blocked I/O 200 MB 500 MB
GH SM is IO bound, 8 KB Page Size, Blocked I/O
BNL SM GH HH BNL SM GH HH
HDD
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17. Joins are I/O Bound? (SSD)
8 KB Page Size, Blocked I/O
Buffer Pool Size
200 MB
500 MB
CPU time
I/O time
Join Time (sec)
1.78
1.09
0.70
1.35
1.69
0.64
1.79
0.8
8 KB, Blocked I/O 200 MB 500 MB
GH SM is IO bound, 8 KB Page Size, Blocked I/O
BNL SM GH HH BNL SM GH HH
SSD
Joins may become CPU-bound sooner
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18. Effect of Varying the Page Size (SSD)
More details in our DaMoN’09 paper
Effect of Varying the Page Size (SSD)
500 MB Buffer Pool, Blocked I/O
BNL SM GH HH
CPU time
I/O time
Join Time (sec)
bigger
Page size (KB)
When using Blocked I/O, the page size has
a small impact on join performance
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19. Conclusions / Future Work
▶ Traditional Join optimizations continue to be important with flash SSDs
Blocked I/O dramatically improves join performance
Buffer allocation strategy has an impact on join performance
It is even more critical to consider both CPU and I/O costs
▶ Future Work
Expand the range of hardware, and consider other HDD-based configurations
Derive detailed cost models for existing join algorithms, and explore the optimal buffer allocations for flash SSDs
Provide better insights for join performance on flash SSDs
Before you go to develop new join algorithms on flash SSDs, m ake sure that you can get large performace improvements with basic join algorithms by using many traditional join optimizations.
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