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PebblesDB : building key-value stores using fragmented log-structured merge trees
Key-value stores such as LevelDB and RocksDB offer excellent write throughput, but suffer high write amplification. The write amplification problem is due to the Log-Structured Merge Trees data structure that underlies these key-value stores. To remedy this problem, this thesis presents a novel data structure that is inspired by Skip Lists, termed Fragmented Log- Structured Merge Trees (FLSM). FLSM introduces the notion of guards to organize logs (sstables or files containing the data on storage), and avoids rewriting data in the same level. Theoretically, we show how FLSM can address the problem of write amplification. We build PebblesDB, a high-performance key-value store, by modifying HyperLevelDB to use the FLSM data structure. We evaluate PebblesDB using micro-benchmarks and show that for write-intensive workloads, PebblesDB reduces write amplification by 2.4-3x compared to RocksDB, while increasing write throughput by 6.7x. We evaluate PebblesDB extensively under a variety of benchmarks, workload patterns, and environmental factors and analyze how it performs in different scenarios. We modify two widely-used NoSQL stores, MongoDB and HyperDex, to use PebblesDB as their underlying storage engine. Evaluating these applications using the YCSB benchmark shows that throughput is increased by 18-105% when using PebblesDB (compared to their default storage engines) while write IO is decreased by 35-55%.Computer Science
Prefix Siphoning: Exploiting LSM-Tree Range Filters For Information Disclosure (Full Version)
Key-value stores typically leave access control to the systems for which they
act as storage engines. Unfortunately, attackers may circumvent such read
access controls via timing attacks on the key-value store, which use
differences in query response times to glean information about stored data.
To date, key-value store timing attacks have aimed to disclose stored values
and have exploited external mechanisms that can be disabled for protection. In
this paper, we point out that key disclosure is also a security threat -- and
demonstrate key disclosure timing attacks that exploit mechanisms of the
key-value store itself.
We target LSM-tree based key-value stores utilizing range filters, which have
been recently proposed to optimize LSM-tree range queries. We analyze the
impact of the range filters SuRF and prefix Bloom filter on LSM-trees through a
security lens, and show that they enable a key disclosure timing attack, which
we call prefix siphoning. Prefix siphoning successfully leverages benign
queries for non-present keys to identify prefixes of actual keys -- and in some
cases, full keys -- in scenarios where brute force searching for keys (via
exhaustive enumeration or random guesses) is infeasible.Comment: Full version of USENIX ATC'23 pape
Biofuel as an alternative shipping fuel : technological, environmental and economic assessment
© Royal Society of Chemistry 2019Fossil derived fuels available for application within the maritime sector have been dominated by heavy fuel oil (HFO), which is conventionally used in low speed (main) engines, and more refined fuels such as marine diesel oil (MDO), which is consumed in fast or medium speed engines. However, increasing fuel costs and regulatory pressure such as the restrictions placed on sulphur content have increased interest in the use of alternative fuels. A number of alternative fuels have been identified and may be viable for use within the maritime sector including straight vegetable oil (SVO) as an alternative to HFO in low speed engines, biodiesel to replace MDO/MGO in low to medium speed engines and bio-liquefied natural gas (bio-LNG) in gas engines using LNG. The potential sources of biomass feedstocks, conversion pathways and technologies are identified. The key parameters limiting their potential application are examined, in particular, availability, technological development, technical integration, and operational consequences. A proposed solution to overcome these limitations is recommended. The effective implementation of these strategies will enable the more widespread use of biofuels in marine applications, significantly reducing emissions from ships and improving global air quality and also protecting the ecological environment.Peer reviewe
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