Evaluation of performance and space utilisation when using snapshots in the ZFS and Hammer file systems

Abstract

Modern file systems implements snapshots, or read-only point-in-time representations of the file system. Snapshots can be used to keep a record of the changes made to the data, and improve backups. Previous work had shown that snapshots decrease read- and write performance, but there was an open question as to how the number of snapshots affect the file system. This thesis studies this on the ZFS and Hammer file systems. The study is done by running a series of benchmarks and creating snapshots of each file system. The results show that performance decreases significantly on both ZFS and Hammer, and ZFS becomes unstable after a certain point; there is a steep decrease in performance, and increase in latency and the variance of the measurements. The performance of ZFS is significantly lower than on Hammer, and the performance decrease is higher. On space utilisation, the results are linear for ZFS, up to the point where the system turns unstable. The results are not linear on Hammer, but more work is needed to reveal by which function.