University of North Carolina

"With the Solid Data solid-state disk I've been able to scale the performance of the server in the face of tremendous growth and improve quality of service for client connections."

Chris Colomb
System Administrator
University of North Carolina

Overview
E-mail system architectures rely heavily on storage I/O, as their function is to quickly process incoming network packets and convert them to files on disk. The University of North Carolina (UNC) was facing a problem in scaling their campus mailserver to support a rapidly growing user base and dramatic increase in mail volume. The University's IMAP (Internet Message Access Protocol) server contains a file - called the mailboxes file - that maintains a sorted list of each mailbox on the server, and keeps the state of every user's access control list (ACL). The IMAP server is heavily loaded during peak hours because the ACL tracks which users can access which mailboxes. Write performance is critical, as folders are created and deleted continuously.

The Problem
The mailboxes file was being stored on a conventional RAID system using rotating disks. This approach had been maintained and scaled while the user base grew over twenty-fold. Users started complaining about the slow system response, as the server performance was being bottlenecked by disk I/O. Given the high level of I/O activity to the mailboxes file, it was decided that a much faster storage system was needed.

The Solution
UNC moved the mailboxes file and the subscription information to a non-volatile solid-state disk (Solid Data's 900 Class Solid-State Disk). Solid-State storage is based upon DRAM technology, has no moving parts, and provides dramatically higher I/O performance than rotating disk.

The actual mailboxes file is relatively small, measuring megabytes in size. However, each active user requires storage space related to this file. The amount of storage depends on the number of users and the amount of folder activity. At UNC there is extensive folder activity including listing, creating, deleting, and checking ACLs. With a peak load of 3,000 users, a total of 3 Gbytes of solid-state storage provided the desired performance level.

Results
By using solid-state storage for the mailboxes file, UNC has decreased the average message response time at peak load from more than twenty seconds, to under two seconds. According to Chris Colomb, the University's System Administrator, "Response time is absolutely crucial for interactive use, where people perceive responses beyond two seconds to be slow. With the Solid Data solid-state disk I've been able to scale the performance of the server in the face of tremendous growth and improve quality of service for client connections."

UNC has 51,000 total users, and the system processes over 500,000 incoming messages per day. The University now has nearly 5,000 simultaneous users during peak load periods, and expects higher loads in the coming semesters. It has recently added memory cards to expand the solid-state disk to 5 Gbytes. By including solid-state storage in the architecture, UNC can easily scale its e-mail infrastructure and keep ahead of their expanding workload requirements.

University of North Carolina Case Study PDF: 20 KB (unc.pdf)