Re: Avoiding fragmentation

From:

Owen Jacobson <angrybaldguy@gmail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Fri, 18 Mar 2011 22:46:54 -0400

Message-ID:

<2011031822465481842-angrybaldguy@gmailcom>

On 2011-03-18 15:33:45 -0400, Roedy Green said:

I just notice how when I create files with Java, they often are badly
fragmented, with ten or more fragments.

This could be avoided with a simple strategy. When you open the file
for write you specify the estimated size of the file. The OS then
looks for a continuous hunk of disk space to satisfy. If you guess too
big, you return the tail to the free space pool. If you guess too
small, it adds another extent or two.

You can do that, or something much like it, by building up larger
chunks of the file in memory before writing them to disk (using NIO,
RandomAccessFile, your own buffering, or BufferedOutputStream). There's
no guarantee that the filesystem will fragment less given larger
writes, but a large block written at once at least propagates some size
information to the OS, whereas a small write to a stream indicates
nearly nothing about the volume of data following it.

This isn't really a Java issue, though: the lowest common denominator
for file creation is POSIX-like, which assumes infintely-appendable
files with no fixed size (or size hints). Blame Bell, basically. ;)

-o