Added Alternative layout policies for directories

Submitted by: Kirk McKusick

FAQ/hackers.sgml

@@ -1,4 +1,4 @@
-<!-- $Id: hackers.sgml,v 1.4 1998-09-04 09:00:08 obrien Exp $ -->
+<!-- $Id: hackers.sgml,v 1.5 1998-09-06 10:54:05 wosch Exp $ -->
 <!-- The FreeBSD Documentation Project -->
 
   <sect>
@@ -261,5 +261,31 @@
       appreciate hearing about your driver on
       <tt>&lt;freebsd-hackers@FreeBSD.ORG&gt;</tt>.
 
-  </sect>
 
+    <sect1>
+      <heading>Alternative layout policies for directories</heading>
+
+      <p>
+      In answer to the question of alternative layout policies for
+      directories, the scheme that is currently in use is unchanged
+      from what I wrote in 1983. I wrote that policy for the original
+      fast filesystem, and never revisited it. It works well at keeping
+      cylinder groups from filling up. As several of you have noted,   
+      it works poorly for find. Most filesystems are created from      
+      archives that were created by a depth first search (aka ftw).    
+      These directories end up being striped across the cylinder groups
+      thus creating a worst possible senario for future depth first    
+      searches. If one knew the total number of directories to be      
+      created, the solution would be to create (total / fs_ncg) per    
+      cylinder group before moving on. Obviously, one would have to    
+      create some heuristic to guess at this number. Even using a      
+      small fixed number like say 10 would make an order of magnitude
+      improvement. To differentiate restores from normal operation   
+      (when the current algorithm is probably more sensible), you    
+      could use the clustering of up to 10 if they were all done       
+      within a ten second window. Anyway, my conclusion is that this   
+      is an area ripe for experimentation.</p>
+
+      <p>Kirk McKusick, September 1998</p>
+
+  </sect>