Sharif Digital Repository

Solid-State Drives (SSDs) are replaced to a great portion of Hard Disk Drives (HDDs) in personal computers, servers, and supercomputers due to their high reliability, shock resistance, low power consumption, and high performance. SSDs utilize either NAND or NOR flash chips in the drive backend logic to persistently store user data. Since NAND/NOR flash chips incur from limited number of write endurance and high write/erase operation latency, they use a controller called Flash Translation Layer (FTL) to alleviate these limitations. As the performance of commercially off-the-shelf (COTS) SSDs is tuned for few mainstream applications, COTS SSDs provide limited throughput for variety of... 

With increases in amount of data being processed and generated, the need for highperformance data storage systems is essential. These systems must keep up with request throughputs as high as millions of requests per second. Solid-State Drives (SSD) lack mechanical parts and have lower read and write latency. SSDs have key differences with traditional spinning drives in having a fixed read/write granularity, erase before write and requiring wear-leveling and garbage collection operations. These operations have significant impact on drive performance and also frequency of these operations depends on the workload characteristics, such as randomness. In this thesis, a high-level estimator of SSD... 

With prominent features such as lower power consumption, shock resistance, and smaller size, SSDs are widely being used in portable applications. In addition, SSDs are becoming more popular in data storage systems and servers due to their even access and fast response time. Several efforts have been made in the past few years to improve the performance of SSDs. One common technique to improve performance is designing an Input/Output (IO) scheduler based on SSD organization.
In this thesis, we propose an IO scheduling algorithm to improve both performance and endurance of SSDs. Since in-place update is not allowed in flash chips, erasure before any data overwriting is required. As... 

Storage is well-known as the performance bottleneck in I/O intensive programs such as online transaction processing applications; thus designing a cost-effective high performance disk subsystem is a major concern in enterprise applications. Data tiering is an efficient way to optimize cost, performance, and reliability in storage servers. With the promising advantages of Solid State Drives (SSDs) over Hard Disk Drives (HDDs) such as lower power consumption and higher performance , traditional data tiering techniques should be revisited in order to use SSDs in a more efficient way. This thesis presents a data tiering technique at the Operating System (OS) level to enhance system performance... 

In the past two decades, there has been a significant performance enhancement in processors by leveraging nano-scale semiconductor technologies and micro-architectural techniques. At the same time, there has been a limited performance improvement in storage devices. This performance gap results in a performance bottleneck in computer systems. To fill this gap, Solid-State Disks (SSDs) has been proposed in the previous work. Due to not using mechanical parts, SSDs can provide higher performance and lower power consumption compared to hard disk drives. Typically, SSDs use flash memory chips to store user data. Flash memory has some shortcomings such as limited endurance and low write... 

Deduplication is a data reduction technique, which eliminates redundant data by storing only a single copy of each file or block, along with a reference to the unique copy of data. Deduplication reduces the storage space and bandwidth requirements of data storage systems, and becomes more effective when applied across multiple users. All deduplication techniques use cryptographic algorithms to detect duplication instead of byte-by-byte comparison between input data and all previously stored data. In this manner, the data stream is divided into non-overlapping chunks of data. Then, the hash values of non-overlapping chunks create an exclusive criterion to identify duplicates. In this thesis,... 

With the technological advances in silicon technology, the price of flash based storage devices has significantly fallen and they are now a main choice for mass data storage. Solid state disks are however, still much more expensive per-capacity than hard disks and tiering can be utilized to use them cost-effectively. Tiering attempts to make use of the diversity offered by storage devices and backend configurations to support the diverse needs of I/O workloads. Tiering is generally done on top of static storage hierarchies and as such, moving a data block into a certain tier will dictate its configuration as well. In this research, an architecture is proposed than can independently encode... 

The fastest growing of data in the fields of economic, cultural and scientific quick analysis of this data is one of the needs of users. The inability of the traditional programming model for processing large data makes MapReduce programming model to be introduced. MapReduce model, processing big data in shorter time available. Users to analyze large data processing, rent necessary resources from infrastructure of cloud as one of the most popular are provided. Due to cloud resources acquired and run-time program on selected configurations, the cost of these resources are paid according to their use. So if resources are not proportionate with the needs of the program will be wasting money and... 

The use of raed cache of hard disk or solid state drives has a magnificent effect on their performance, power consumption, and their endurance. However, one of the main problems of their read cache is its low hit ratio. Thus, the read cache must be used more efficiently. Based on previous studies, nearly all of the blocks which are brought to the read cache are zero-reuse or dead-on-arrival, which means that after the blocks enter to the read cache, they are not accessed until they are evicted. Thus, a large portion of the disk read cache is nearly unused because of these dead blocks. In this thesis, we will design a disk read cache which does not allow to the dead blocks to enter the... 

In recent years applying solid state drives in data centers, cloud computing infrastructures and commercial servers have had a considerable growth. Low power consumption, high throughput and resistance to physical damages are some of the advantages of SSDs over HDDs. In recent works the necessity of using an interconnection network as a communication subcomponent between memory flash chips and disk controller is described. Disk mechanisms (mapping, wear-leveling and garbage collection) influence on throughput of the network. Based on this fact in this research an analytical model is provided considering disk mechanisms, flash chip structure and traffic characteristics. The error of the model... 

As a new technology, the use of Solid State Disk (SSD) in storage systems is increasing due to its higher performance and lower power consumption as compared to the traditional Hard Disk Drive (HDD). On the contrary, the SSD technology has two serious shortcomings, i.e., 1) it has a higher Bit Error Rate (BER) as the number of Program/Erase (P/E) cycles increases, and 2) it can endure a specific number of P/E cycles, called the write endurance limitation. To overcome the endurance limitation of SSDs, several solutions have been proposed in previous literature. These solutions can be categorized in two levels: 1) SSD-controller, and 2) RAID–Controller. Moreover, it is necessary to consider... 

Solid State Drives (SSDs) have an electronic architecture based on non-volatile memories. The unique merit of a SSD is its internal parallelism. Thus, adopting SSDs as the storage media in the storage systems in various devices, such as smart phones, personal computers, large workstations, and etc., improves the performance of the device, substantially. However, the conventional host interface protocols in SSDs, i.e. SATA and SAS, have critical limitations, like low bus bandwidth (maximum 12Gbps), and a single shared request queue. With such limitations, the host interface turns into a performance bottleneck in conventional SSDs. Therefore, the high performance provided by such SSDs cannot... 

One of the most intrinsic challenges of flash-based Solid State Drives (SSDs) is erasebefore-write limitation and the limited endurance of flash chips. Wear leveling and garbage collection are two mechanisms implemented in SSD’s controller to enhance endurance and performance. While wear leveling attempts to distribute erasures across all blocks in an even manner, it imposes a new security challenge on SSD,which leads to the presence of invalid data blocks in flash chips. Therefore, some data blocks that are logically deleted by the user are still available in flash chips and can be recovered by software or hardware recovery tools.
In this paper, a new criteria named vulnerability time... 

Storage subsystem is one of the most important components of a computer system. As more valuable information such as bank transactions are being stored in storage subsystems,occurrence of an unrecoverable error can lead to a much bigger problem. A storage subsystem can be divided into software and hardware part. Previous works focused on fault injection in the higher layers of the storage stack and didn’t consider fault injection at the device driver level. In this thesis, the dependability of storage stack is evaluated using fault injection at the device driver layer. Various hardware and software faults that can occur in hard disks and solid state drives are studied and dependability of... 

As a promising replacement for the conventional high-latency and low-throughput HDDs, NAND Flash-based solid state drives have been increasingly used in data center and cloud applications as well as high-end enterprise servers. However, capacity scaling is a controversial challenge of the SSD manufacturers to keep pace with the competitors in the storage market. To date, SSD designs have been largely based on multi-channel bus architecture that confronts serious scalability problems in high-end enterprise SSDs with dozens of Flash memory chips and a gigabyte host interface. This forces the community to rapidly change the bus-based inter-Flash standards to respond to ever increasing... 

In recent years, flash-based Solid State Drives (SSDs), because of their lower power consumption, higher throughput, and resistance against physical damages compared to Hard Disk Drives (HDDs) have confronted ever increasing usage in data center, cloud applications, and enterprise servers. Recently, advantages of using interconnection networks between SSD controller and NAND Flash chips for transferring data and commands have been shown. Interconnection network intrinsically provides many advantages such as maintaining the signal integrity at high frequencies, pipelining support, sharing the communication resources, eliminating communication bottlenecks, and reducing power consumption. The...