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POext4/fsync situation unclear in Android (Java)
 

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    <p>Tim Bray's article <a href="http://android-developers.blogspot.com/2010/12/saving-data-safely.html" rel="noreferrer">"Saving Data Safely"</a> left me with open questions. Today, it's over a month old and I haven't seen any follow-up on it, so I decided to address the topic here.</p> <p>One point of the article is that FileDescriptor.sync() should be called to be on the safe side when using FileOutputStream. At first, I was very irritated, because I never have seen any Java code doing a sync during the 12 years I do Java. Especially since coping with files is a pretty basic thing. Also, the standard JavaDoc of FileOutputStream never hinted at syncing (Java 1.0 - 6). After some research, I figured ext4 may actually be the first mainstream file system requiring syncing. (Are there other file systems where explicit syncing is advised?)</p> <p>I appreciate some general thoughts on the matter, but I also have some specific questions:</p> <ol> <li>When will Android do the sync to the file system? This could be periodic and additionally based on life cycle events (e.g. an app's process goes to the background).</li> <li>Does FileDescriptor.sync() take care of syncing the meta data? That is syncing the directory of the changed file. Compare to FileChannel.force().</li> <li>Usually, one does not directly write into the FileOutputStream. Here's my solution (do you agree?): <code><pre> FileOutputStream fileOut = ctx.openFileOutput(file, Context.MODE_PRIVATE); BufferedOutputStream out = new BufferedOutputStream(fileOut); try { out.write(something); out.flush(); fileOut.getFD().sync(); } finally { out.close(); } </pre></code></li> </ol>
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