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\section*{Data Spooling}
\label{_ChapterStart5}
\index[general]{Data Spooling }
\index[general]{Spooling!Data }
\addcontentsline{toc}{section}{Data Spooling}

Bacula allows you to specify that you want the Storage daemon to initially
write your data to disk and then subsequently to tape. This serves several
important purposes. 

\begin{itemize}
\item It can take a long time for data to come in from the File  daemon during
   an Incremental backup. If it is directly  written to tape, the tape will start
   and stop or shoe-shine  as it is often called causing tape wear. By first
   writing  the data to disk, then writing it to tape, the tape can be  kept in
   continual motion. 
\item While the spooled data is being written to the tape, the  despooling
   process has exclusive use of the tape. This  means that you can spool multiple
   simultaneous jobs to disk,  then have them very efficiently despooled one at a
   time  without having the data blocks from several jobs  intermingled, thus
   substantially improving the time needed  to restore files.  
\item Writing to a tape can be slow. By first spooling your  data to disk, you
   can often reduce the time the File  daemon is running on a system, thus
   reducing downtime,  and/or interference with users. 
\end{itemize}

Data spooling is exactly that "spooling". It is not a way to first write a
"backup" to a disk file and then to a tape. When the backup has only been spooled to disk,
it is not complete yet and cannot be restored until it is written to tape. In a
future version, Bacula will support writing a backup to disk then later {\bf
Migrating} or {\bf Copying} it to a tape. 

The remainder of this chapter explains the various directives that you can use
in the spooling process. 
\label{directives}

\subsection*{Data Spooling Directives}
\index[general]{Directives!Data Spooling }
\index[general]{Data Spooling Directives }
\addcontentsline{toc}{subsection}{Data Spooling Directives}

The following directives can be used to control data spooling. 

\begin{itemize}
\item To turn data spooling on/off at the Job level in  the Job resource in
   the Director's conf file (default  {\bf no}).

{\bf SpoolData = yes|no} 

\item To override the Job specification in a Schedule Run  directive in the
   Director's conf file.

{\bf SpoolData = yes|no} 

\item To limit the maximum total size of the spooled data  for a particular
   device. Specified in the Device  resource of the Storage daemon's conf file
   (default  unlimited).

{\bf Maximum Spool Size = size}
   Where size is a the maximum spool size for all jobs  specified in bytes. 

\item To limit the maximum total size of the spooled data  for a particular
   device for a single job. Specified  in the Device Resource of the Storage
   daemon's conf  file (default unlimited).

{\bf Maximum Job Spool Size = size}
   Where size is the maximum spool file size for a single  job specified in
   bytes. 

\item To specify the spool directory for a particular device.  Specified in
   the Device Resource of the Storage daemon's conf  file (default, the working
   directory).

{\bf Spool Directory = directory} 
\end{itemize}

\label{warning}

\subsection*{!!! MAJOR WARNING !!!}
\index[general]{WARNING! MAJOR }
\index[general]{ MAJOR WARNING  }
\addcontentsline{toc}{subsection}{MAJOR WARNING !!!}

Please be very careful to exclude the spool directory from any backup,
otherwise, your job will write enormous amounts of data to the Volume, and
most probably terminate in error. This is because in attempting to backup the
spool file, the backup data will be written a second time to the spool file,
and so on ad infinitum. 

Another advice is to always specify the maximum spool size so that your disk
doesn't completely fill up. In principle, data spooling will properly detect a
full disk, and despool data allowing the job to continue. However, attribute
spooling is not so kind to the user. If the disk on which attributes are being
spooled fills, the job will be canceled. In addition, if your working 
directory is on the same partition as the spool directory, then Bacula jobs
will fail possibly in bizarre ways when the spool fills.

\label{points}
\subsection*{Other Points}
\index[general]{Points!Other }
\index[general]{Other Points }
\addcontentsline{toc}{subsection}{Other Points}

\begin{itemize}
\item When data spooling is enabled, Bacula automatically  turns on attribute
   spooling. In other words, it also  spools the catalog entries to disk. This is
   done so  that in case the job fails, there will be no catalog  entries
   pointing to non-existent tape backups. 
\item Attribute despooling is done at the end of the job, as  a consequence,
   after Bacula stops writing the data to  the tape, there may be a pause while
   the attributes are sent  to the Directory and entered into the catalog before 
   the job terminates. 
\item Attribute spool files are always placed in the  working directory. 
\item When Bacula begins despooling data spooled to disk, it  takes exclusive
   use of the tape. This has the major  advantage that in running multiple
   simultaneous jobs at  the same time, the blocks of several jobs will not be 
   intermingled. 
\item It probably does not make a lot of sense to enable data  spooling if you
   are writing to disk files. 
\item It is probably best to provide as large a spool file as  possible to
   avoid repeatedly spooling/despooling. Also,  while a job is despooling to
   tape, the File daemon must wait  (i.e. spooling stops for the job while it is 
   despooling).  
\item If you are running multiple simultaneous jobs, Bacula  will continue
   spooling other jobs while one is despooling  to tape, provided there is
   sufficient spool file space. 
\end{itemize}
