Transaction Log maintenance on Mirror database
We are doing log backups on the principal database, and my question is what are the gotchas with doing a log backup on a mirror?


You can't do a log backup on the mirror database.


  At least one full database backup from the mirror has to be completed before doing a log backup, in that case are there special options that need to be used because it is a mirror?


You can't do a full database backup on the mirror database either.

Take this for instance:  I have Server1 which houses the principal database AdventureWorks2012, and I have Server2 which contains the mirror.  Here's what happens when I try to run backups on the mirror database (on Server2):

use master;
go

backup database AdventureWorks2012
to disk = 'c:\sqlserver\AW_mirror.bak';
go



  Msg 954, Level 14, State 1, Line 2
  The database "AdventureWorks2012" cannot be opened. It is acting as a mirror database.
  Msg 3013, Level 16, State 1, Line 2
  BACKUP DATABASE is terminating abnormally.


backup log AdventureWorks2012
to disk = 'c:\sqlserver\AW_mirror.trn';
go



  Msg 954, Level 14, State 1, Line 1
  The database "AdventureWorks2012" cannot be opened. It is acting as a mirror database.
  Msg 3013, Level 16, State 1, Line 1
  BACKUP LOG is terminating abnormally.


Take a look at this Database Mirroring FAQ by Robert Davis.  I will quote him regarding this operation and the mirror database transaction log maintenance:


  When you backup the log on the principal, the virtual log files (individual units within the log file) are marked as re-writable. The same VLF’s are marked as re-writable in the mirror log file as well. The VLF status is mirrored on the database.


So there you have it.  If you have transaction log backups on the principal then similar log reuse behavior will be mirrored to its partner database.