File Modification While Tar is in Progress

[Manish Kathuria <manish@xxxxxxxxxxxx>]

I am using tar to take backups of multiple files and directories and 
some of the files are about 500Mb. What happens when archiving of a 
large file using tar (plus compression switches) is in progress and the 
file is altered by some other program ? What do I need to do to make 
sure that my archive is not corrupted ? For example, the large file in 
question could be an IMAP mailbox on a Mail Server where I cannot afford 
to stop either the MTA or the IMAP server to prevent the file from 
getting altered.

Thanks,

Manish Kathuria