Calculating how much data an IBM MQ queue file can store

The amount of data that can be stored on a queue is limited by the size of the individual blocks the queue is divided into. Use MQSC commands to confirm block size and granularity, and check the size of a queue file.

Before you begin

Note: The steps in this task require you to run MQSC commands:

On AIX®, Linux®, and Windows, you issue MQSC commands from a runmqsc command prompt. See Running MQSC commands interactively under runmqsc and Running MQSC commands from text files under runmqsc.
On IBM® i, you create a list of commands in a Script file, then run the file by using the STRMQMMQSC command. See Administration using MQSC commands on IBM i.

Procedure

Confirm block size and granularity.
The default block size is 512 bytes. To support queue files greater than two terabytes, the queue manager will need to increase the block size.

The block size is automatically calculated when you configure the MAXFSIZE for a queue, but the revised block size cannot be applied to the queue if the queue already has messages on it. Once a queue is empty, the queue manager automatically modifies the block size to support the configured MAXFSIZE.

The DISPLAY QSTATUS command has a new attribute, CURMAXFS, that allows you to confirm that a queue has been modified to use a new block size.
In the following example, the CURMAXFS value of 4177920 confirms that the queue file is currently able to grow to approximately four terabytes in size. If the value of MAXFSIZE configured on the queue is greater than the value of CURMAXFS, then the queue manager is still waiting for the queue to be emptied before it reconfigures the block size of the queue file.
```
DISPLAY QSTATUS(LARGEQUEUE)  CURMAXFS
     2 : DISPLAY QSTATUS(LARGEQUEUE)  CURMAXFS
AMQ8450I: Display queue status details
  QUEUE(LARGEQUEUE)                         TYPE(QUEUE)
  CURMAXFS(4177920)                         CURDEPTH(100000)
```
Check the size of a queue file.
You can display the current size of a queue file on disk, in megabytes, using the CURFSIZE attribute in the DISPLAY QSTATUS command. This can be useful on platforms such as the IBM MQ Appliance, where it is not possible to access the file system directly.
```
DISPLAY QSTATUS(SMALLQUEUE)  CURFSIZE
     1 : DISPLAY QSTATUS(SMALLQUEUE)  CURFSIZE
AMQ8450I: Display queue status details
  QUEUE(SMALLQUEUE)                         TYPE(QUEUE)
  CURDEPTH(4024)                            CURFSIZE(10)
```
Note: When a queue has messages removed from it, the CURFSIZE attribute does not decrease immediately.
Typically, space in a queue file is only released when no applications have the queue open and there are no in-doubt messages stored on the queue. Any necessary truncation or compaction of a queue file that is loaded by the queue manager occurs during checkpoint, queue manager shutdown or whilst recording a media image of the queue.