kes Implement grow tool to grow a file for testing very large databases.

[bacula/docs] / docs / developers / catalog.tex

diff --git a/docs/developers/catalog.tex b/docs/developers/catalog.tex
index 6a5d96a2de4873efdbe9e2e372bda67d151dbaa0..74599151e6e0630f716ebe9e9fbf5d592c0f39ee 100644 (file)
--- a/docs/developers/catalog.tex
+++ b/docs/developers/catalog.tex
@@ -498,10 +498,20 @@ Job  } \\
 
 \end{longtable}
 
-The {\bf JobMedia} table contains one entry for each volume written for the
-current Job. If the Job spans 3 tapes, there will be three JobMedia records,
-each containing the information to find all the files for the given JobId on
-the tape. 
+The {\bf JobMedia} table contains one entry at the following: start of
+the job, start of each new tape file, start of each new tape, end of the
+job.  Since by default, a new tape file is written every 2GB, in general,
+you will have more than 2 JobMedia records per Job.  The number can be
+varied by changing the "Maximum File Size" specified in the Device 
+resource.  This record allows Bacula to efficiently position close to
+(within 2GB) any given file in a backup.  For restoring a full Job,
+these records are not very important, but if you want to retrieve
+a single file that was written near the end of a 100GB backup, the   
+JobMedia records can speed it up by orders of magnitude by permitting
+forward spacing files and blocks rather than reading the whole 100GB
+backup.
+        
+
 
 \