As long as the path you are putting in your monitor stanza matches the path and pattern that would include those older files, it should read them in.
So, for example, if you have files in /ijk/lmn/ called opt.log, opt1.log, opt2.log qrs.log, qrs1.log, etc, then your monitor stanza should be something like...
[monitor:///ijk/lmn/]
...in order to pick up all of the files inside of the lmn directory. In that example of files I list above, and the example of the monitor stanza you stated in your last comment, you would be missing all of the older files because you have it watching only a specific filename (/ijk/lmn/opt.log).
If you have files that are perhaps log rotated to a different filename, like adding a .1, .2, .3, etc to the end, or have a dated file that rotates on a regular basis, then you'll need to account for those file name differences.
something like
[monitor:///ijk/lmn/opt*.log]
may be appropriate.
The default for ignoreOlderThan is to be unset, meaning there is no threshold and no files are ignored for modification time reasons. So otherwise, your stanza settings should be fine.
You may also need to consider if the older files may appear to be identical to the new files as far as their content goes. By default, the input only performs CRC checks against the first 256 bytes of a file. This behavior prevents the input from indexing the same file twice, even though you might have renamed it, such as log rotation, for example. So if your log files happen to have a long 'header' in them that is the same for every file, they may be seen as 'identical'. in that case you may need to set a crcSalt attribute on your stanza to have it consider the files different if the filenames are different. in that case you'd use
crcSalt = <source> (literally the string '<source>').
Just be careful if your filenames start out with otp.log and when rolled end up becoming a file called opt1.log, for example, because then your files will be indexed a second time when they role.
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