An I/O scheduler for dual-partitioned tapes

For a long time, tapes have had a single logical partition that was efficiently operated by dedicated software in batch mode. Nowadays, after the introduction of the LTO 5 standard, tapes support logical partitioning and, with the arrival of the Linear Tape File System (LTFS), their contents are exposed through the file system interface as regular files and directories. As a consequence, tape medium can now be accessed by concurrent processes that may create files in parallel. This creates two potential problems: interleaved data blocks and expensive partition switches. This paper presents the design and implementation of Unified, an I/O scheduler for LTFS that addresses these problems. We observe the effectiveness of delayed writes on decreased file fragmentation and the reduction of partition switches with the use of buffering and of redundant file copies. We also find out that software-based read prefetching, commonly used to manage disk devices, does not improve read times on tapes, but rather introduces potential overhead.With the techniques described in this paper, the Unified scheduler allows tape operations to be performed close to the raw hardware speed.