When calculating the required disk space for servers, consider the maximum number of messages likely to be stored on the server at any one time. Because messages may be stored on target computer, source computer, and any MSMQ routing server in between the two, it is difficult to anticipate message storage requirements. However, if you configure MSMQ independent clients to use InRSs or OutRSs, or sites to use site gates, you will need to allow for additional message space on those MSMQ servers. Also, those servers that support mobile independent clients will likely need additional disk space.
Because journaled messages are stored until an administrator removes them, or application reads the journal queue, you must allow for additional disk space when journaling messages.
The amount of disk space used for messages depends on the message type (recoverable or express), message size, and messaging volume. Recoverable messages are allocated in 4K blocks. Each message using approximately 100 bytes of MSMQ overhead (depending on the message properties). This yields approximately 256 messages per megabyte (MB) of disk space for messages under approximately 4,000 bytes.
Because express messages are written to disk when insufficient memory is available to store them all, you must also allow for their disk space usage. Because express messages are stored in 64 byte blocks, disk space is used more efficiently. For example, you can store approximately 1,280 1,024-byte recoverable messages in 5 MB, or you can store approximately 5,120 1,024-byte express messages in 5 MB.
Note MSMQ allocates disk space for both express and recoverable messages in 4 MB files. This ensures that enough disk space will be available when memory is flushed, and messages are written to disk.
As a rule, you should base your message type selection on business needs, not disk space requirements. For more information on MSMQ message types, see "MSMQ Message Delivery," in Chapter 1, Understanding MSMQ.