The mboxlist directory is highly I/O intensive but not very large. The mboxlist directory contains the databases that are used by the stores and their transaction logs. Because of its high I/O activity, and due to the fact that the multiple files that constitute the database cannot be split between different file systems, you should place the mboxlist directory on its own stripe or volume in large deployments. This is also the most likely cause of a loss of vertical scalability, as many procedures of the Message Store access the databases. For highly active systems, this can be a bottleneck. Bottlenecks in the I/O performance of the mboxlist directory decrease not only the raw performance and response time of the store but also impact the vertical scalability. For systems with a requirement for fast recovery from backup, place this directory on Solid State Disks (SSD) or a high performance caching array to accept the high write rate that an ongoing restore with a live service will place on the file system.