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Effect of readahead and file system block reallocation for LBCAS (LoopBack Content Addressable Storage)

Disk prefetching, known as readahead, arranges its window size by the rate of cache hit. Fewer readaheads of large window can hide the slow I/O, especially it is effective for virtual block device of virtual machine. High cache hit ratio is achieved by increasing locality of reference, namely, file system block reallocation based on an access profile. The reallocation doesn't reserve block contiguousness in a file but it is effective for archival virtual block device like a content addressable storage.

We have developed a data block reallocation tool for ext2/3, called "ext2/3optimizer". The relocation is applied to Linux booting on KVM virtual machine with a virtual disk called "LBCAS (LoopBack Content Addressable Storage)". We confirmed keeping large window of readahead and fewer access requests to LBCAS. The results are useful not only for OS migration but for normal Linux booting. This work is conducted in collaboration with Toshiki Yagi (AIST), Kengo Iijima (AIST), Nguyen Anh Quynh (AIST), and Yoshihito Watanabe (Alpha Systems Inc.)

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