08-12-2009, 01:40 PM
Seltzer, M. I., Ganger, G. R., McKusick, M. K., Smith, K. A., Soules, C. A., and Stein, C. A. Journaling versus soft updates: asynchronous meta-data protection in file systems. In Proceedings of the Annual Conference on USENIX Annual Technical Conference (San Diego, California, June 18 - 23, 2000).
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A very nice paper on various issues concerning journalling and soft updates.
Maintaining the integrity of meta-data is critical to the consistency of a file system. Synchronous updates result in low throughput. Hence, asynchronous update schemes, such as soft updates and journaling, are invented to make meta-data updates asynchronous without compromising file system consistency. This paper compares journaling and soft updates in terms of performance as well as semantic difference -- "the durability of meta-data operations", "the status of the file system after a reboot and recovery", "the guarantees made about the data in files after recovery", and "the ability to provide atomicity".
In addition to providing an excellent survey on journaling and soft updates, this paper shows empirical results on several typical file system organizations, as listed below:
FFS
FFS with asynchronous metadata updates
Soft updates
LFFS with a log file
LFFS with wafs (write-ahead file system) on one disk, synchronous meta-data updates
LFFS with wafs on one disk, asynchronous meta-data updates
LFFS with wafs on two disks, synchronous meta-data updates
LFFS with wafs on two disks, asynchronous meta-data updates
The workloads used in the evaluation include microbenchmarks, SSH, Netnews, and SDET (time sharing). The empirical data provides many insights into the two techniques under comparison, and the authors' comments on the results are enlightening. But some interesting data points go without being explained (e.g., why soft-updates peaks at 4KB in Figure 2?), probably due to space limit or lack of certainty.
* * *
A very nice paper on various issues concerning journalling and soft updates.
Maintaining the integrity of meta-data is critical to the consistency of a file system. Synchronous updates result in low throughput. Hence, asynchronous update schemes, such as soft updates and journaling, are invented to make meta-data updates asynchronous without compromising file system consistency. This paper compares journaling and soft updates in terms of performance as well as semantic difference -- "the durability of meta-data operations", "the status of the file system after a reboot and recovery", "the guarantees made about the data in files after recovery", and "the ability to provide atomicity".
In addition to providing an excellent survey on journaling and soft updates, this paper shows empirical results on several typical file system organizations, as listed below:
FFS
FFS with asynchronous metadata updates
Soft updates
LFFS with a log file
LFFS with wafs (write-ahead file system) on one disk, synchronous meta-data updates
LFFS with wafs on one disk, asynchronous meta-data updates
LFFS with wafs on two disks, synchronous meta-data updates
LFFS with wafs on two disks, asynchronous meta-data updates
The workloads used in the evaluation include microbenchmarks, SSH, Netnews, and SDET (time sharing). The empirical data provides many insights into the two techniques under comparison, and the authors' comments on the results are enlightening. But some interesting data points go without being explained (e.g., why soft-updates peaks at 4KB in Figure 2?), probably due to space limit or lack of certainty.