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<html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Optimized Blanking Stategy</title><link rel="stylesheet" type="text/css" href="styles.css"><meta name="generator" content="DocBook XSL Stylesheets V1.76.1"><link rel="home" href="index.html" title="Cedar Backup Software Manual"><link rel="up" href="ch05.html" title="Chapter 5. Configuration"><link rel="prev" href="ch05s06.html" title="Configuring your Writer Device"><link rel="next" href="ch06.html" title="Chapter 6. Official Extensions"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Optimized Blanking Stategy</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch05s06.html">Prev</a> </td><th width="60%" align="center">Chapter 5. Configuration</th><td width="20%" align="right"> <a accesskey="n" href="ch06.html">Next</a></td></tr></table><hr></div><div class="sect1" title="Optimized Blanking Stategy"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="cedar-config-blanking"></a>Optimized Blanking Stategy</h2></div></div></div><p>
         When the optimized blanking strategy has not been configured, Cedar
         Backup uses a simplistic approach: rewritable media is blanked at the
         beginning of every week, period.  
      </p><p>
         Since rewritable media can be blanked only a finite number of times
         before becoming unusable, some users &#8212; especially users of
         rewritable DVD media with its large capacity &#8212; may prefer to
         blank the media less often.
      </p><p>
         If the optimized blanking strategy is configured, Cedar Backup will
         use a blanking factor and attempt to determine whether future backups
         will fit on the current media.  If it looks like backups will fit,
         then the media will not be blanked.
      </p><p>
         This feature will only be useful (assuming single disc is used for the
         whole week's backups) if the estimated total size of the weekly backup
         is considerably smaller than the capacity of the media (no more than
         50% of the total media capacity), and only if the size of the backup
         can be expected to remain fairly constant over time (no frequent rapid
         growth expected).
      </p><p>
         There are two blanking modes: daily and weekly.  If the weekly
         blanking mode is set, Cedar Backup will only estimate future capacity
         (and potentially blank the disc) once per week, on the starting day of
         the week.  If the daily blanking mode is set, Cedar Backup will
         estimate future capacity (and potentially blank the disc) every time
         it is run.  <span class="emphasis"><em>You should only use the daily blanking mode in
         conjunction with daily collect configuration, otherwise you will risk
         losing data.</em></span>
      </p><p>
         If you are using the daily blanking mode, you can typically set the
         blanking value to 1.0.  This will cause Cedar Backup to blank the
         media whenever there is not enough space to store the current day's
         backup.
      </p><p>
         If you are using the weekly blanking mode, then finding the correct
         blanking factor will require some experimentation.  Cedar Backup
         estimates future capacity based on the configured blanking factor.
         The disc will be blanked if the following relationship is true:
      </p><pre class="screen">
bytes available / (1 + bytes required) &#8804; blanking factor
      </pre><p>
         Another way to look at this is to consider the blanking factor as a
         sort of (upper) backup growth estimate:
      </p><pre class="screen">
Total size of weekly backup / Full backup size at the start of the week
      </pre><p>
         This ratio can be estimated using a week or two of previous backups.
         For instance, take this example, where March 10 is the start of the
         week and March 4 through March 9 represent the incremental backups
         from the previous week:
      </p><pre class="screen">
/opt/backup/staging# du -s 2007/03/*
3040    2007/03/01
3044    2007/03/02
6812    2007/03/03
3044    2007/03/04
3152    2007/03/05
3056    2007/03/06
3060    2007/03/07
3056    2007/03/08
4776    2007/03/09
6812    2007/03/10
11824   2007/03/11
      </pre><p>
         In this case, the ratio is approximately 4:
      </p><pre class="screen">
6812 + (3044 + 3152 + 3056 + 3060 + 3056 + 4776) / 6812 = 3.9571
      </pre><p>
         To be safe, you might choose to configure a factor of 5.0.  
      </p><p>
         Setting a higher value reduces the risk of exceeding media capacity
         mid-week but might result in blanking the media more often than is necessary.  
      </p><p>
         If you run out of space mid-week, then the solution is to run the
         rebuild action.  If this happens frequently, a higher blanking factor
         value should be used.
      </p></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch05s06.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ch05.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ch06.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Configuring your Writer Device </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 6. Official Extensions</td></tr></table></div></body></html>