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PMTA knowledge base›Metric§ 5.3

Spool capacity

spool recipient capacity
How many recipients fit

How many recipients the spool holds now and how many it can hold at most.

The “now and maximum” pair is a ready safety margin. A node with nine tenths occupied behaves normally right up to the first jam at a large receiver.

⚠️ The Hub reads both values and does not use them as a margin: node sizing is not calculated here.

The main trap is taking a snapshot for a history. The metric describes the moment of polling. Everything that happened between two measurements and managed to end will not get into either of them. For events there is accounting, for state — these numbers.
What the Hub does. It shows it live: the node monitor reads both values and displays spool occupancy in percent, but the poll does not put them into the time series.

⚠️ These numbers are not used as a margin: node sizing is not calculated here.

Recommended values

What to compare with:

Value Compared with
current value the usual one for this node, not the neighboring one
peak the ceiling, if there is one
growth the growth rate, not the absolute number

Fleet nodes are not directly comparable: they have different hardware, different addresses and a different mix of receivers. It makes sense to compare a node with itself over time.

The comparison advice is ours. From the manual here come the metric’s composition and its units of measurement.
Example configuration
# the metric comes in the status snapshot
curl -s "http://127.0.0.1:8080/status?format=json"pmta show status
The highlighted line returns the snapshot as a structure — that is how the Hub’s polling reads it. The line below prints the same thing in the console.
yellow — — directive names, blue — blocks. They are highlighted the same way in the text of the card.
Compatibility
PowerMTA 6.0
described in the manual of this version · platform: any
The vendor does not state the first version for every directive — there is no note here. The name also appears in the 5.0 edition of the manual.
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Related entries

2 relations

Works together with

Queue depthQueue depth is exactly what fills this ceiling. § 5.3

Applies to

How to turn a sending plan into hardwareA limit the first chapter’s sizing estimate does not mention: it is expressed in recipients. inference

Referenced from

Spool filesFile occupancy against the spool ceiling in recipients. § 5.3

How to check on the node
The metric is viewed together with the neighboring ones — on its own any number means little:
pmta show status
pmta show queues --sort=rcpt --maxitems=20
The first prints all node metrics at once. The second breaks down by queue what the first gives as one number — and that is usually where you can see which queue made it up.
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What else to read

3 articles

◇The activation key is a prerequisite for installation, not a step after itWithout an activation key from SparkPost support the engine neither installs nor runs, and the license tier sets the outbound connection ceiling: on Standard everything above 200 is ignored.◇Cloud restrictionsClouds restrict outbound mail on port 25 until you ask them to lift the limit, and mailbox providers treat their address ranges with suspicion.◇How to turn a sending plan into hardwareTwo constants for sizing any node: about 2 KB of memory per queued recipient and spool space proportional to the total size of the messages.

In the Hub
Cost of a mistake
safe: a wrong value breaks nothing
Scenarios
Capacity planning