Don't Thrash: How to Cache Your Hash on Flash

This paper presents new alternatives to the well-known Bloom filter data structure. The Bloom filter, a compact data structure supporting set insertion and membership queries, has found wide application in databases, storage systems, and networks. Because the Bloom filter performs frequent random reads and writes, it is used almost exclusively in RAM, limiting the size of the sets it can represent. This paper first describes the quotient filter, which supports the basic operations of the Bloom filter, achieving roughly comparable performance in terms of space and time, but with better data locality. Operations on the quotient filter require only a small number of contiguous accesses. The quotient filter has other advantages over the Bloom filter: it supports deletions, it can be dynamically resized, and two quotient filters can be efficiently merged. The paper then gives two data structures, the buffered quotient filter and the cascade filter, which exploit the quotient filter advantages and thus serve as SSD-optimized alternatives to the Bloom filter. The cascade filter has better asymptotic I/O performance than the buffered quotient filter, but the buffered quotient filter outperforms the cascade filter on small to medium data sets. Both data structures significantly outperform recently-proposed SSD-optimized Bloom filter variants, such as the elevator Bloom filter, buffered Bloom filter, and forest-structured Bloom filter. In experiments, the cascade filter and buffered quotient filter performed insertions 8.6-11 times faster than the fastest Bloom filter variant and performed lookups 0.94-2.56 times faster.Comment: VLDB201

Rural carers online: A feasibility study

Objective: To test the feasibility (for a potential randomised controlled trial) of a computer intervention for improving social interaction and promoting the mental health of rural carers. Design: The study combined pre- and post-intervention measures with interviews to determine the feasibility of the intervention and the acceptability of the study design to participants. The intervention consisted of providing 14 rural carers with computers and a 4-week training program on basic computer skills, using email and the Internet. Setting: The study was conducted in a rural community setting. Participants: The carers were 12 women and two men, aged from 50 to 81 years, with an average of 65.5 years. Main outcome measures: Measures of social isolation (UCLA Loneliness Scale), depression (Geriatric Depression Scale), carer burden (Zarit Burden Interview) and computer confidence were taken at baseline and at a 3-month follow-up. Interviews were completed at follow-up to discuss outcomes of the study. A focus group discussion was conducted with 11 participants to discuss the study and resolve computer issues. Results: Most carers reported increased confidence in email and Internet use. There was improvement for most participants in depressive symptoms and social isolation, but little change in carer burden. Participants identified many social benefits associated with the computer intervention, such as intergenerational connection, community building, skills and confidence and preparation for the future. Conclusion: The intervention was found to be practical and acceptable for a group of older carers. It was concluded that it would be feasible to conduct a large randomised controlled trial of the intervention. © Journal compilation © 2008 Blackwell Publishing