- <para>The <literal>filefrag</literal> utility reports the extent of fragmentation in a given file. Initially, <literal>filefrag</literal> attempts to obtain extent information using <literal>FIEMAP ioctl</literal>, which is efficient and fast. If <literal>FIEMAP</literal> is not supported, then <literal>filefrag</literal> uses <literal>FIBMAP</literal>.</para>
+ <para>The <literal>filefrag</literal> utility reports the extent of fragmentation in a given file.
+ The <literal>filefrag</literal> utility obtains the extent information
+ from Lustre files using the <literal>FIEMAP ioctl</literal>, which is
+ efficient and fast, even for very large files.</para>
+ <para>In default mode <footnote>
+ <para>The default mode is faster than the verbose/extent mode since
+ it only counts the number of extents.</para>
+ </footnote>, <literal>filefrag</literal> prints the number of physically
+ discontiguous extents in the file. In extent or verbose mode, each
+ extent is printed with details such as the blocks allocated on each OST.
+ For a Lustre file system, the extents are printed in device offset order
+ (i.e. all of the extents for one OST first, then the next OST, etc.),
+ not file logical offset order. If the file logical offset order was
+ used, the Lustre striping would make the output very verbose and
+ difficult to see if there was file fragmentation or not.</para>