Expand the reserved space section. Explain how the amount of reserved

space can affect performance.

Submitted by:	David Schultz <dschultz@uclink.Berkeley.EDU>

1 files changed, 12 insertions, 4 deletions

diff --git a/sbin/tunefs/tunefs.8 b/sbin/tunefs/tunefs.8
index 89a6d3f5b8eb..c160e13edf35 100644
--- a/sbin/tunefs/tunefs.8
+++ b/sbin/tunefs/tunefs.8
@@ -100,13 +100,21 @@ Specify the expected average file size.
 Specify the percentage of space held back
 from normal users; the minimum free space threshold.
 The default value used is 8%.
-This value can be set to zero, however up to a factor of three
-in throughput will be lost over the performance obtained at a 10%
-threshold.
+Note that lowering the threshold can adversely affect performance:
+.Pp
+.Bl -bullet
+.It
 Settings of 5% and less force space optimization to
 always be used which will greatly increase the overhead for file
 writes.
-Note that if the value is raised above the current usage level,
+.It
+The filesystem's ability to avoid fragmentation will be reduced
+when the total free space, including the reserve, drops below 15%.
+As free space approaches zero, throughput can degrade by up to a
+factor of three over the performance obtained at a 10% threshold.
+.El
+.Pp
+If the value is raised above the current usage level,
 users will be unable to allocate files until enough files have
 been deleted to get under the higher threshold.
 .It Fl n Ar enable | disable