27.8.5 imsimta counters

The MTA accumulates message traffic counters based upon the Mail Monitoring MIB, RFC 1566 for each of its active channels. The channel counters are intended to help indicate the trend and health of your e-mail system. Channel counters are not designed to provide an accurate accounting of message traffic. For precise accounting, instead see MTA logging as discussed in Chapter 25, Managing Logging.

The MTA channel counters are implemented using the lightest weight mechanisms available so that they cause as little impact as possible on actual operation. Channel counters do not try harder: if an attempt to map the section fails, no information is recorded; if one of the locks in the section cannot be obtained almost immediately, no information is recorded; when a system is shut down, the information contained in the in-memory section is lost forever.

The imsimta counters -show command provides MTA channel message statistics (see below). These counters need to be examined over time noting the minimum values seen. The minimums may actually be negative for some channels. A negative value means that there were messages queued for a channel at the time that its counters were zeroed (for example, the cluster-wide database of counters created). When those messages were dequeued, the associated counters for the channel were decremented and therefore leading to a negative minimum. For such a counter, the correct “absolute” value is the current value less the minimum value that counter has ever held since being initialized.

Channel          Messages    Recipients    Blocks 
-------          --------    ----------    ------- 
tcp_local
   Received       29379       79714      982252                (1)
   Stored            61         113       -2004                (2)
   Delivered      29369       79723      983903 (29369 first time)  (3)
   Submitted      13698       13699       18261                (4)
   Attempted          0           0           0                (5)
   Rejected           1          10           0                (6)
   Failed           104         104        4681                (7)

   Queue time/count        16425/29440 = 0.56                  (8)
   Queue first time/count  16425/29440 = 0.56                  (9)

   Total In Assocs           297637
   Total Out Assocs           28306

1) Received is the number of messages enqueued to the channel named tcp_local. That is, the messages enqueued (E records in the mail.log* file) to the tcp_local channel by any other channel.

2) Stored is the number of messages stored in the channel queue to be delivered.

3) Delivered is the number of messages which have been processed (dequeued) by the channel tcp_local. (That is, D records in the mail.log* file.) A dequeue operation may either correspond to a successful delivery (that is, an enqueue to another channel), or to a dequeue due to the message being returned to the sender. This will generally correspond to the number Received minus the number Stored.

The MTA also keeps track of how many of the messages were dequeued upon first attempt; this number is shown in parentheses.

4) Submitted is the number of messages enqueued (E records in the mail.log file) by the channel tcp_local to any other channel.

5) Attempted is the number of messages which have experienced temporary problems in dequeuing, that is, Q or Z records in the mail.log* file.

6) Rejected is the number of attempted enqueues which have been rejected, that is, J records in the mail.log* file.

7) Failed is the number of attempted dequeues which have failed, that is, R records in the mail.log* file.

8) Queue time/count is the average time-spent-in-queue for the delivered messages. This includes both the messages delivered upon the first attempt, see (9), and the messages that required additional delivery attempts (hence typically spent noticeable time waiting fallow in the queue).

9) Queue first time/count is the average time-spent-in-queue for the messages delivered upon the first attempt.

Note that the number of messages submitted can be greater than the number delivered. This is often the case, since each message the channel dequeues (delivers) will result in at least one new message enqueued (submitted) but possibly more than one. For example, if a message has two recipients reached via different channels, then two enqueues will be required. Or if a message bounces, a copy will go back to the sender and another copy may be sent to the postmaster. Usually that will be two submissions (unless both are reached through the same channel).

More generally, the connection between Submitted and Delivered varies according to type of channel. For example, in the conversion channel, a message would be enqueued by some other arbitrary channel, and then the conversion channel would process that message and enqueue it to a third channel and mark the message as dequeued from its own queue. Each individual message takes a path:

elsewhere -> conversion E record Received
conversion -> elsewhere E record Submitted
conversion              D record Delivered

However, for a channel such as tcp_local which is not a “pass through,” but rather has two separate pieces (slave and master), there is no connection between Submitted and Delivered. The Submitted counter has to do with the SMTP server portion of the tcp_local channel, whereas the Delivered counter has to do with the SMTP client portion of the tcp_local channel. Those are two completely separate programs, and the messages travelling through them may be completely separate.

Messages submitted to the SMTP server:

tcp_local -> elsewhere E record Submitted

Messages sent out to other SMTP hosts via the SMTP client:

elsewhere -> tcp_local E record Received
tcp_local              D record Delivered

Channel dequeues (delivers) will result in at least one new message enqueued (submitted) but possibly more than one. For example, if a message has two recipients reached via different channels, then two enqueues will be required. Or if a message bounces, a copy will go back to the sender and another copy may be sent to the postmaster. Usually that will be reached through the same channel.

27.8.5.1 Implementation on UNIX and NT

For performance reasons, a node running the MTA keeps a cache of channel counters in memory using a shared memory section (UNIX) or shared file-mapping object (NT). As processes on the node enqueue and dequeue messages, they update the counters in this in-memory cache. If the in-memory section does not exist when a channel runs, the section will be created automatically. (The imta start command also creates the in-memory section, if it does not exist.)

The command imta counters -clear or the imta qm command counters clear may be used to reset the counters to zero.