Reducing Disk I/O of Messaging Systems Kristina Ninan

Email Protocols Simple Mail Transfer Protocol (SMTP) Post Office Protocol 3 (POP3) Internet Mail Access Protocol (IMAP)

SMTP Mail Transfer Agent (MTA) Used between sender computer and sender mail server Used between sender mail server and receiver mail server

POP3 Mail Access Agent (MAA) Used between receiver mail server and receiver computer Allows receiver to access messages through email client by downloading messages to receiver computer

Role of Disk I/O in Email Outgoing messages are saved temporarily to a disk queue until they can be sent “Deliver” command from the messaging server reads a message file from here and then tries to send it “SMTP Data” command from the messaging server receives the incoming message, creates a new file on disk, and writes message to this file After successful delivery, message object is deleted off disk, and can be moved to a different section of memory (i.e. sent folder)

Issues Deliver and SMTP Data commands take up significantly more time than all other commands Inefficient lookup time if all files are saved in a single directory with subdirectories

Issues Most email systems store one message per file → high overhead Causes fragmentation because files are stored in non-adjacent blocks, which increases seek time and decreases throughput Deleting files after delivery also increases the disk I/O and therefore overhead and fragmentation

Issues Majority of emails are small Separate files for each message cause a large number of small files Causes large number of small file writes Small files can’t make full use of bandwidth Disk I/O = one of the primary performance bottlenecks

Potential Optimizations Need to optimize storage management Reduce seek time by storing files in a multiple hash directory - introduced by Qmail Qmail also write files to the directory in a round robin manner

SmartQ Proposed by a research group at the University of Electronic Science & Technology of China Aims to decrease the overhead in file management - especially file creation and deletion Main idea: Implement disk queue using pre-assigned continuous disk space Adjacent messages stored in adjacent blocks Policies: Lazy Gathering Write Policy and Sequential Grouping Prefetching

SmartQ - Lazy Gathering Write Policy Accumulate multiple messages in a large buffer in one large read and write them all at once into the disk queue

SmartQ - Lazy Gathering Write Policy Takes full advantage of disk bandwidth Reduces disk seek time and rotational latency Multiple adjacent messages are written to continuous blocks

SmartQ - Sequential Grouping Prefetching Policy Multiple adjacent messages are fetched into buffer in one read

SmartQ - Sequential Grouping Prefetching Policy Takes full advantage of bandwidth Number of message objects read is determined by the buffer size and can also be determined from the MIN table in memory

SmartQ - Message Index Node (MIN) To improve read performance Store a MIN table in memory which indexes messages waiting to be delivered Allows message to be quickly located by mapping from the message ID to its location Deleting a message after successful delivery = deleting MIN and setting delete flag in memory, which is write of size 1 byte Decreases disk I/O Decreases fragmentation

IBM User Specified Conditions Disk Management In 2012 IBM proposed an improvement based on user specified conditions If enabled, user can specify disk usage conditions based on information associated with incoming mail This information can include the sender’s email address, the email header, attachment type, etc If maximum quota is met, or a condition is violated, incoming message is trashed without being downloaded

IBM User Specified Conditions Disk Management

IBM User Specified Conditions Disk Management Use counters to keep track of the size of the mail saved to disk Reduce number of reads when trying to determine if mailbox is full/can hold incoming message Increases server overhead but decreases disk I/O

Sources Wei, Qingsong & Lu, Xianliang & Ren, Liyong & Zhou, Xu. (2003). A novel disk queue to reduce disk I/O of messaging system. Operating Systems Review. 37. 55-60. 10.1145/881783.881790. French, Stephen M. et. al. Managing Email Disk Usage Based on User Specified Condiditions. 18 Oct. 2012. D. Schuff, O. Turetken, J. D'Arcy and D. Croson, "Managing E-Mail Overload: Solutions and Future Challenges," in Computer, vol. 40, no. 2, pp. 31-36, Feb. 2007. doi: 10.1109/MC.2007.65 http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4085619 &isnumber=4085604

Sources McDonald, Alistair, et al. Linux Email Set up and Run a Small Office Email Server. Packt Publishing, 2005. Curtis, Derek. “Derek Curtis.” Email Server, Group Chat, Help Desk and Web Analytics Software, SmarterTools Inc., 16 May 2013, www.smartertools.com/blog/2013/05/16-improving-disk-io-and-overall-performance-for-your-mail-server-part-1. Curtis, Derek. “Derek Curtis.” Email Server, Group Chat, Help Desk and Web Analytics Software, SmarterTools Inc., 23 May 2013, www.smartertools.com/blog/2013/05/23-improving-disk-io-and- overall-performance-for-your-mail-server-part-2.

Sources Curtis, Derek. “Derek Curtis.” Email Server, Group Chat, Help Desk and Web Analytics Software, SmarterTools Inc., 30 May 2013, www.smartertools.com/blog/2013/05/30-improving-disk-io-and-overall-performance-for-your-mail-server-part-3. Unknown. “Difference Between SMTP and POP3.” Tech Differences, TechDifferences, 7 July 2017, techdifferences.com/difference-between-smtp-and-pop3.html. Celine. "Difference Between SMTP and POP." DifferenceBetween.net. June 29, 2017 < http://www.differencebetween.net/technology/internet/difference-between-smtp-and-pop/ >.

Sources Unknown. “How the SMTP Service Works.” How the SMTP Service Works, Wrox Press, 1998, msdn.microsoft.com/en- us/library/aa480390.aspx.

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