Chapter 6. Tune Performance

When sendmail is installed with near-default settings, it provides excellent email services for most machines. But when installed to service high loads, high volumes, or high rates, special tuning becomes a requirement.

What’s New with V8.13

There are a few new items in V8.13 that affect performance tuning. They are described in other chapters but referenced here. In this chapter, we augment some of the knowledge imparted in the third edition of the sendmail book.

  • The RequiresDirfsync option (Section 24.1.12 [V8.13]) turns off the REQUIRES_DIR_FSYNC (3.4.47[3ed]) compile-time macro’s setting at runtime. Turning off directory fsyncs increases performance—but at (possibly) increased risk.

  • The existing SuperSafe option (24.9.107[3ed]) now accepts a new PostMilter setting that delays fsync( )ing the df file until after all Milters have reviewed the message. This improves performance when a great deal of email is rejected by Milters that review the message body.

  • The Timeout.queuereturn.dsn (Section 24.1.15[V8.13]) and Timeout.queuewarn.dsn (Section 24.1.16 [V8.13]) options have been added. Use them to lower bounce timeouts, and thereby to create less congested queues and increase performance.

  • Some sites have developed delivery agents that receive messages using SMTP over standard input/output. Such delivery agents use the P=[LPC] equate (20.5.11[3ed]) to achieve this effect. Beginning with V8.13, sendmail enables connection caching (24.7.5[3ed]) for such delivery agents, thereby increasing delivery performance.

Queue Disk Mounts

Although this is not a V8.13 improvement, you can safely increase the performance of your queue disks under Solaris 7 and above, and other operating systems by mounting them with the following mount(1) options:

logging,noatime

Here, the logging causes transactions (such as creating and deleting files) to be stored in a log before they are applied to the disk. Once a transaction is logged, it can be applied to the underlying disk layout later. This speeds up disk I/O and can help a machine to reboot faster.

The noatime prevents inodes from being updated each time a file is read. This eliminates a disk write that has no significant value. The speed increase will be most noticeable when many queued files are being retried in parallel.

One or the other of these mount(1) options may not be available with your operating system. See your online documentation to find out which you can use.

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