Subject: Re: vfs.lfs.pagetrip--unexpected result
To: Blair Sadewitz <>
From: Thor Lancelot Simon <>
List: current-users
Date: 12/11/2006 00:19:31
On Mon, Dec 11, 2006 at 12:15:16AM -0500, Blair Sadewitz wrote:
> Now, this is where it gets strange.  To arrive at a value for
> vfs.lfs.pagetrip, I was told to divide the bandwidth of my disk (in
> the case of the ccd, it is ~162000000 by 4096 and then divide that by
> four, producing a result that should be bigger than your chosen
> segment size.  However, if I simply divide the bandwidth by 4096 (size
> of one page) and set vfs.lfs.pagetrip to, for example, 39950, bonnie++
> results tend toward one third better write performance, and other
> improvements all around.  If anyone has the hardware to create and
> benchmark LFS filesystems to investigate this, espically on raidframe
> or ccd.

You underestand that bandwidth is not the only important measurement of
disk performance, right?  If you set the pagetrip value so that writes
are started only when a full second of data is available to be written,
you're going to significantly increase the _latency_ of the filesystem.

Where you set the balance point best will depend on your application but
for most applications except for video writing (only, with no reads) the
value you're suggesting is too high.

Thor Lancelot Simon	                           
  "The liberties...lose much of their value whenever those who have greater
   private means are permitted to use their advantages to control the course
   of public debate."					-John Rawls