Disk Performance of Copy-On-Write Snapshot Logical Volumes

Data Snapshot technology is a standard feature of modern storage systems. Most such systems use copy-on-write techniques to manage snapshot data in order to optimize storage space requirements for maintaining history data. Copy-on-write methods tend to write data out-of-place at a location which may be far away from the original location of the data on the disk. This phenomenon gradually leads to fragmentation of the ondisk snapshot data and degradation in the disk I/O performance. This work analyzes Logical Volume Manager’s (LVM2) snapshot technology and studies the effect of copyon-write on the on-disk placement of the snapshot data. Based on these findings, we propose new disk space allocation and data placement techniques for snapshot volumes in order to reduce physical distance between related blocks and improve disk access performance. A prototype is implemented and its performance is compared with the original LVM2 implementation in order to measure the effectiveness of the proposed schemes. The new schemes tend to perform better than the old LVM2 ranging from 18% to 40% at the cost of some performance penalty for first time writes in some cases.