11.3 Using Multiple Queue Directories

Beginning with V8.10, sendmail allows the use of multiple queue directories. These multiple queue directories take two forms:

  • More than one queue directory can be specified, possibly on separate disks, into which all the qf, df, and xf files are placed.

  • Any queue directory can have a subdirectory named qf, and/or df, and/or xf, in which sendmail stores the corresponding qf, df, and xf files.

11.3.1 Multiple Queue Directories

V8.10 sendmail offers the ability to distribute queued messages across multiple directories. In general, this is a good idea. If, for example, a high volume of email is stressing your current disk, you can improve efficiency by using multiple queue directories spread over multiple disks and controllers.

To illustrate, we will set up a machine that has three brand-new disks to use as multiple queue directories. The disks have already been formatted and a filesystem has been placed on each. We next create directories on which to mount them:

# mkdir /var/queues /var/queues/q.1 /var/queues/q.2 /var/queues/q.3
# chmod 700 /var/queues /var/queues/q.?

Because of the way multiple queue directories are implemented inside sendmail, the queue directory names must differ only in their suffixes, hence the trailing 1, 2, and 3. First the directories are created with mkdir(1) or a symbolic link, then the permission on each is reduced to readable and writable only by root for security reasons. Note that these are the permissions after all queue disks are mounted.

Next, arrange for the disks to be mounted by placing the appropriate entries in /etc/fstab or /etc/vfstab. Here, we illustrate with the partial contents of /etc/fstab for Linux:

/dev/hda2         /var/queues/q.1     ext2     defaults    1 1
/dev/hdb1         /var/queues/q.2     ext2     defaults    1 1
/dev/hdc1         /var/queues/q.3     ext2     defaults    1 1

Note that we are mounting a separate disk on each queue directory. Your disk device names will doubtless differ, and you can use any directory locations and names you wish. Note that after you mount the disks, you might need to change the permissions again to 700 for each mount point.

The idea is to prepare the directories for use as multiple queue directories first, and after that, to modify the configuration file so that sendmail can use those queue directories:


Here, the QUEUE_DIR mc configuration macro is given the value /var/queues/q.*, which will become the value for the QueueDirectory option. The trailing * character is a literal asterisk (not a wildcard character) and must appear as a suffix, in the last position of the path specification. It tells sendmail to use all the queue directories that begin with the path /var/queues/q. and end with any other characters. In our example, sendmail will match /var/queues/q.1, /var/queues/q.2, and /var/queues/q.3.

It is not strictly necessary to mount a disk on each queue directory. If the directory name is a symbolic link to another directory, sendmail will use that other directory as a queue directory. The only requirement is that the other directory have as restrictive set of permissions as the original queue has.

In Section 6.5 of Chapter 6, we show how to use multiple queue directories to improve overall performance. Printing multiple queue directories

After you have configured for multiple queue directories, you will find there is a small difference in the way various versions of sendmail print the queue contents. Prior to V8.10 sendmail, the heading for a queue listing printed like this:

Mail queue is empty        when nothing is queued (pre-V8.10)
Mail queue (1 request)     when one message is queued (pre-V8.10)

Starting with V8.10 sendmail, that heading now looks like this:

/var/spool/mqueue is empty  when nothing is queued (V8.10 and above)
/var/spool/mqueue (1 request)   when one message is queued (V8.10 and above)

The full pathname of the queue is printed, regardless of whether you are running multiple queue directories. This behavior is beneficial when running multiple queue directories because it lets you know which queue directory contains what mail:

/var/spool/mqueues/q.1 is empty
                /var/spool/mqueues/q.2 (1 request)
----Q-ID---- --Size-- -----Q-Time----- ------------Sender/Recipient------------
dB9Fdaa06420     4567 Thu Dec  9 07:39 you@your.domain
                Total Requests: 1

From this output, it is clear that q.1 is empty and q.2 contains a single message. Unfortunately, the two headings indent differently but this is easily gotten used to.

In the previous output, also notice that when multiple queue directories are printed, a trailing line is printed after all queue directories are printed that shows the total of all messages in all queue directories. If you run dozens or hundreds of queue directories, you might find it useful to summarize the number of queued messages like this:

% mailq -OMaxQueueRunSize=1 | tail -1
                Total Requests: 41291

The expression -OMaxQueueRunSize=1 (MaxQueueRunSize) causes sendmail to process each queue directory extremely fast, regardless of how many messages are queued in each. Processing multiple queue directories

When sendmail processes multiple queue directories it processes them in parallel. That is, it forks and runs a queue processing child of sendmail for each, all of which run at the same time. The maximum number of sendmail queue processors run is limited by the MaxDaemonChildren option (MaxDaemonChildren). If that limit is reached before all the queue directories can be parallel-processed, sendmail will remember where it stopped and perform the next run starting from where the prior run left off.

The only exception to this behavior occurs when queue processing with the -v (verbose) command-line switch. When -v is combined with -q, processing is always sequential. That is, one queue directory is processed at a time, and the next is not begun until the first finishes. The -v allows you to watch the queue being processed, so it makes sense that you would want to watch only one queue directory at a time.

11.3.2 Using qf, df, and xf Subdirectories

Beginning with V8.10, sendmail allows the qf, df, and xf files to reside in separate directories. One advantage to this is that it produces directories that are one-third smaller. Another advantage is that each part can reside on a separate disk for further performance enhancements.

This feature is enabled by simply creating the appropriately named subdirectories, or symbolic links, in each queue directory. The names of those subdirectories or symbolic links are the literals qf, df, and xf. But be aware that you should not create those directories or links when mail is already queued. If you do, that queued mail will disappear from sendmail's view and will never be delivered. If you need to make the change while mail is queued, first stop sendmail, then execute the following commands and restart sendmail:

# mkdir df qf xf 
# chmod 700 df qf xf 
# mv df?* df/              if mail is already queued 
# mv qf?* qf/              if mail is already queued 
# mv xf?* xf/              if mail is already queued 

Here, we first create the new subdirectories in the queue directory. Then we reduce their permissions to the narrow ones that match the queue directory. Finally, if queued mail already existed in the queue directory, we move that mail into the new subdirectories where sendmail will find it.

Because xf files are empty for all successfully delivered mail, there is a penalty for creating and deleting those files just because they might be needed. When performance is of concern, you can either mount a memory filesystem on the xf subdirectory, or replace the xf subdirectory with a symbolic link to a directory on a memory filesystem. In the following, we show a /etc/fstab file for a SunOS machine that uses the direct-mount approach:

/dev/sd0g  /var/spool/mqueue/df        4.2 rw        1 4
/dev/sd2g  /var/spool/mqueue/qf        4.2 rw        1 2
swap       /var/spool/mqueue/xf        tmp rw        0 0

See Section 6.5 for a description of how to use a different type of disk for each part, and how performance is impacted by such choices.

An artifact of using qf, df, and xf subdirectories is seen when printing the queue. The df directory is always the one listed:

/var/spool/mqueue/df is empty

    Part I: Build and Install
    Part II: Administration
    Part III: The Configuration File