Browsing by Author "Chang, Da-Wei"

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A Lightweight Buffer Overflow Protection Mechanism with Failure-Oblivious Capability
(2009)
Lee, Tz-Rung
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Chiu, Kwo-Cheng
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Chang, Da-Wei
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資訊工程學系
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Department of Computer Science
Buffer overflow has become a major source of network security vulnerability. Traditional schemes for detecting buffer overflow attacks usually terminate the attacked service, degrading the service availability. In this paper, we propose a lightweight buffer overflow protection mechanism that allows continued network service. The proposed mechanism allows a service program to reconfigure itself to identify and protect the vulnerable functions upon buffer overflow attacks. Protecting only the Vulnerable functions, instead of the whole program, keeps the runtime overhead small. Moreover, the mechanism adopts the idea Of failure-oblivious computing to allow service programs to execute through memory errors caused by the attacks once the Vulnerable functions have been identified, eliminating the need of restarting the service program upon further attacks to the vulnerable functions. We have applied the mechanism on five Internet servers. The experiment results show that the mechanism has little impact oil the runtime performance.
Closed-loop seizure control on epileptic rat models
(2011-08-01)
Liang, Sheng-Fu
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Liao, Yi-Cheng
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Shaw, Fu-Zen
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Chang, Da-Wei
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Young, Chung-Ping
In this paper numerous alternative treatments in addition to pharmacological therapy are proposed for their use in epileptic patients. Epileptic animal models can play a crucial role in the performance evaluation of new therapeutic techniques. The objective of this research is to first develop various epileptic rat models; second, develop a portable wireless closed-loop seizure controller including on-line seizure detection and real-time electrical stimulation for seizure elimination; and third, apply the developed seizure controller to the animal models to perform on-line seizure elimination. The closed-loop seizure controller was applied to three Long-Evans rats with spontaneous spike-wave discharges (non-convulsive) and three Long-Evans rats with epileptiform activities induced by pentylenetetrazol (PTZ) injection (convulsive) for evaluation. The seizure detection accuracy is greater than 92% (up to 99%), and averaged seizure detection latency is less than 0.6 s for both spontaneous non-convulsive and PTZ-induced convulsive seizures. The average false stimulation rate is 3.1%. Near 30% of PTZ-induced convulsive seizures need more than two times of 0.5 s electrical stimulation for suppression and 90% of the non-convulsive seizures can be suppressed by only one 0.5 s electrical stimulation.
Extending Lifetime and Reducing Garbage Collection Overhead of Solid State Disks with Virtual Machine Aware Journaling
(2011)
Huang, Ting-Chang
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Chang, Da-Wei
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資訊工程學系
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Department of Computer Science
Virtualization is becoming widely deployed in commercial servers. In our previous study, we proposed Virtual Machine Aware journaling (VMA journaling), a file system journaling approach for virtual server environments. With reliable VMM and hardware subsystems, VMA journaling eliminates journal writes while ensuring file system consistency and data integrity in virtual server environments, allowing it to be an effective alternative to traditional journaling approaches in these environments. In recent years, solid-state disks (SSDs) have shown their potential as a replacement of traditional magnetic disks in commercial servers. In this paper, we demonstrate the benefits of VMA journaling on SSDs. We compare the performance of VMA journaling and three traditional journaling approaches (i.e., the three journaling modes of the ext3 journaling file system) in terms of lifetime and garbage collection overhead, which are two key performance metrics for SSDs. Since a Flash Translation Layer (FTL) is used in an SSD to emulate traditional disk interface and the SSD performance highly depends on the FTL being used, three state-of-the-art FTLs (i.e., FAST, SuperBlock FTL and DFTL) are implemented for performance evaluation. The performance results show that, traditional full data journaling could reduce the lifetime of the SSD significantly. VMA journaling extends the SSD lifetime under full data journaling by up to 86.5%. Moreover, GC overhead is reduced by up to 80.6% when compared to the full data journaling approach of ext3. Finally, VMA journaling is effective under all the FTLs. These results demonstrate that VMA journaling is effective in SSD-based virtual server environments.
Improving Flash Translation Layer Performance by Supporting Large Superblocks
(2010-05-01)
Lin, Pei-Kuan
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Chiao, Mong-Ling
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Chang, Da-Wei
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資訊工程學系
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Department of Computer Science
A Flash Translation Layer (FTL) provides a block device interface on top of flash memory to support existing disk based file systems. Due to the erase-before-write feature of flash memory, an FTL usually performs out-of-place updates and uses a garbage collection procedure to reclaim stale data. Superblock FTL is one of the well-known FTLs, which achieves good performance in terms of both garbage collection overhead and RAM usage. The use of fine-grained mapping information allows a logical page within a superblock, i.e., a set of contiguous logical blocks, to be placed at any page offset of the physical blocks allocated for that superblock. In addition, the fine-grained mapping information is stored in the spare area of NAND flash memory to reduce the RAM usage. Generally, the FTL performance improves with larger superblocks. However, the spare area management approach of the FTL prevents it from supporting large superblocks. In this paper, we propose an FTL that incorporates a novel spare area management approach to enable the support of large superblocks. The simulation results from four traces show that the proposed FTL reduces the garbage collection overhead by up to 89%, when compared to the Superblock FTL. In addition, it results in shorter response time(1).
Kernel support for zero-loss Internet service restart
(2007-07-10)
Chang, Da-Wei
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Tsai, Chuan-Ming
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Li, Wei-Kou
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Lee, Tzu-Rung
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資訊工程學系
Owing to long serving time and huge numbers of clients, Internet services can easily suffer from transient faults. Although restarting a service can solve this problem, information of the on-line requests will be lost owing to the service restart, which is unacceptable for many commercial or transaction-based services. In this paper, we propose an approach to achieve the goal of zero-loss restart for Internet services. Under this approach, a kernel subsystem is responsible for detecting the transient faults, retaining the I/O channels of the service, and managing the service restart How. In addition, some straightforward modifications to the service should be made to take advantage of the kernel support. To demonstrate the feasibility of our approach, we implemented the subsystem in the Linux kernel. Moreover, we modified a Web server and a CGI program to take advantage of the kernel support. According to the experimental results, our approach incurs little runtime overhead (i.e. less than 3.2%). When the service crashes, it can be restarted quickly (i.e. within 210 mu s) with no information loss. Furthermore, the performance impact due to the service crash is small. These results show that the approach can efficiently achieve the goal of zero-loss restart for Internet services. Copyright (C) 2006 John Wiley & Sons, Ltd.
Live Demonstration: A Portable Closed-loop Seizure Controller in Freely Moving Rats
(2012)
Liang, Sheng-Fu
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Shaw, Fu-Zen
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Chang, Da-Wei
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Young, Chung-Ping
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Wang, Yu Lin
nDriver: Online driver replacement for increasing operating system availability
(2007-07-01)
Chang, Da-Wei
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Huang, Zhi-Yuan
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Chang, Ruei-Chuan
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資訊工程學系
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Department of Computer Science
Device drivers are the most unreliable part of an operating system. In this paper, we propose a framework called nDriver. Based on the design diversity concept, it uses multiple implementations of a device driver to survive from driver faults. Once a fault happens in a driver, nDriver can dynamically replace the faulty driver with another implementation, instead of allowing the faulty driver to crash the system. The unique features of nDriver are as follows. First, it can detect two major kinds of driver faults, the exception and blocking faults. Second, the requests issued to the driver will not be lost due to the driver replacement. Third, the driver replacement is transparent to all the other kernel subsystems. Fourth, nDriver requires no modification to the existing operating system or driver codes. The major contribution of this work is that nDriver implements the concept of design diversity at the device driver layer. Moreover, it achieves the goal of seamless driver replacement and improves operating system availability without modifying the existing operating system or driver codes. We implemented nDriver as a kernel module in Linux. Currently, it can recover the system from faults in network device drivers. However, the mechanisms can be adapted to other module-based device drivers with a slight extension. According to the performance evaluation, the overhead of nDriver is no more than 3.5% and the recovery time is quite small. This indicates that nDriver is an efficient mechanism to increase the availability of an operating system.
ROSE: A Novel Flash Translation Layer for NAND Flash Memory Based on Hybrid Address Translation
(2011-06-01)
Chiao, Mong-Ling
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Chang, Da-Wei
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資訊工程學系
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Department of Computer Science
A Flash Translation Layer (FTL) provides a block device interface on top of flash memory to support disk-based file systems. Due to the erase-before-write feature of flash memory, an FTL usually performs out-of-place updates and uses a cleaning procedure to reclaim stale data. A hybrid address translation (HAT)-based FTL combines coarse-grained and fine-grained address translation to achieve good performance while keeping the size of the mapping information small. In this paper, we propose a new HAT-based FTL, called ROSE, which includes three novel techniques for reducing the cleaning cost. First, it reduces high-cost reclamation by preventing data in an entire-block sequential write from being placed into multiple physical blocks while eliminating the cleaning cost resulting from mispredicting random or semisequential writes as sequential ones. Second, it uses a merge-aware cleaning policy that considers both the block age and the merge cost in a HAT-based FTL for improving the cleaning efficiency. Third, it delays the erasure of obsolete blocks and reuses their free pages for buffering more writes. Simulation results show that the proposed FTL outperforms existing HAT-based FTLs in terms of both cleaning cost and flash write time by up to 47 times and 1.6 times, respectively.
TESA: a Temporal and Spatial Information Aware Writeback Policy for Home Network-Attached Storage Devices
(2013-02-01)
Huang, Ting-Chang
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Chang, Da-Wei
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資訊工程學系
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Department of Computer Science
Home Network-Attached Storage (NAS) provides an easy way for data sharing and backup among multiple consumer electronic devices in home networks. Because of large capacity and cost effectiveness, disks are widely adopted in home NAS devices. In addition, data writes are common for many home NAS devices since these devices are usually used for data storage and backup. A writeback policy selects which dirty buffers are to be flushed to the disk, which is critical to the system performance under write-intensive workloads. In this paper, an intelligent writeback policy for home NAS called TESA is proposed. In contrast to most existing writeback policies, TESA considers both temporal and spatial information of the dirty buffers to improve the writeback performance. Considering the temporal information helps to reduce the frequency of writing back recently used dirty buffers, and hence leads to reduced write traffic. Considering the spatial information causes a reduction in the overall seek time and rotation delay of the dirty buffer writeback. The TESA writeback policy was implemented on a NAS evaluation board running Linux kernel. The performance results shows that TESA yields a significant performance improvement (i.e., up to 33.1%) over the original writeback policy of Linux. Moreover, TESA outperforms the policy that considers only the temporal information of the dirty buffers by up to 21.0%, and it also has up to 10.9% performance improvement over a previous policy that considers both the temporal and spatial information(1).
TridentFS: a hybrid file system for non-volatile RAM, flash memory and magnetic disk
(2016-03)
Huang, Ting-Chang
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Chang, Da-Wei
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資訊工程學系
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Department of Computer Science
A hybrid file system with high flexibility and performance, called Trident file system (TridentFS), is proposed to manage three types of storage with different performance characteristics, that is, Non-Volatile RAM (NVRAM), flash memory and magnetic disk. Unlike previous NVRAM-based hybrid file systems, novel techniques are used in TridentFS to improve the flexibility and performance. TridentFS is flexible by the support of various forms of flash memory and a wide range of NVRAM size. The former is achieved on the basis of the concept of stackable file systems, and the latter is achieved by allowing data eviction from the NVRAM. TridentFS achieves high performance by keeping hot data in the NVRAM and allowing data evicted from the NVRAM to be parallel distributed to the flash memory and disk. A data eviction policy is proposed to determine the data to be evicted from the NVRAM. Moreover, a data distribution algorithm is proposed to effectively leverage the parallelism between flash memory and disk during data distribution. TridentFS is implemented as a loadable module on Linux 2.6.29. The performance results show that it works well for both small-sized and large-sized NVRAM, and the proposed eviction policy outperforms LRU by 27%. Moreover, by effectively leveraging the parallelism between flash memory and disk, the proposed data distribution algorithm outperforms the RAID-0 and a size-based distribution method by up to 471.6% and 82.6%, respectively. By considering the data size and performance characteristics of the storage, the proposed data distribution algorithm outperforms the greedy algorithm by up to 15.5%. Copyright (c) 2014 John Wiley & Sons, Ltd.
Virtual machine support for zero-loss Internet service recovery and upgrade
(2007-11-10)
Chang, Da-Wei
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Hsieh, Cheng-En
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Chen, Yan-Pai
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Chiu, Kwo-Cheng
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資訊工程學系
Software is critical for Internet service availability since an Internet service may become unavailable due to software faults or software maintenance. In this paper, we propose a framework to allow zeroloss recovery and online maintenance for Internet services. The framework is based on the virtual machine (VM) technology and a connection migration technique called FT-TCP. It can recover transient application/operating system faults and it allows fault recovery and online maintenance on a single host. The framework substantially enhances FT-TCP so that it can be run efficiently in the VM environment. Specifically, we propose techniques to reduce the inter-VM switches and communication. Moreover, we propose service-specific optimizations to reduce the recovery time of FT-TCP. Finally, the framework provides an interface for the service designers to implement more service-specific optimizations. The framework was implemented by modifying an open source VM monitor, Xen, and the Linux kernel running on top of Xen. The effectiveness and efficiency of the framework were evaluated by running two Internet services, WWW proxy and FTP, on the proposed framework and measuring the impact on their performance. According to the experimental results, our approach causes only slight performance overhead (i.e. less than 4%) and memory overhead (i.e. less than 750 KB) for both the services. Copyright (C) 2007 John Wiley & Sons, Ltd.
VM aware journaling: improving journaling file system performance in virtualization environments
(2012-03-01)
Huang, Ting-Chang
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Chang, Da-Wei
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資訊工程學系
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Department of Computer Science
Journaling file systems, which are widely used in modern operating systems, guarantee file system consistency and data integrity by logging file system updates to a journal, which is a reserved space on the storage, before the updates are written to the data storage. Such journal writes increase the write traffic to the storage and thus degrade the file system performance, especially in full data journaling, which logs both metadata and data updates. In this paper, a new journaling approach is proposed to eliminate journal writes in server virtualization environments, which are gaining in popularity in server platforms. Based on reliable hardware subsystems and virtual machine monitor (VMM), the proposed approach eliminates journal writes by retaining journal data (i.e. logged file system updates) in the memory of each virtual machine and ensuring the integrity of these journal data through cooperation between the journaling file systems and the VMM. We implement the proposed approach in Linux ext3 in the Xen virtualization environment. According to the performance results, a performance improvement of up to 50.9journaling approach of ext3 due to journal write elimination. In metadata-write dominated workloads, this approach could even outperform the metadata journaling approaches of ext3, which do not guarantee data integrity. These results demonstrate that, on virtual servers with reliable VMM and hardware subsystems, the proposed approach is an effective alternative to traditional journaling approaches. Copyright (c) 2011 John Wiley & Sons, Ltd.