Once again throw out the calculation of si_iosize_max and set it to 64K.

Although it can go higher, it is not safe to so do on arrays with many
members.  Compromise by adding a tunable, "hw.aac.iosize_max" that can be
set at boottime.  Also document in the aac(4) manpage.

MFC after:	4 weeks

1 files changed, 12 insertions, 0 deletions

diff --git a/share/man/man4/aac.4 b/share/man/man4/aac.4
index 3988c4d..22167b9 100644
--- a/share/man/man4/aac.4
+++ b/share/man/man4/aac.4
@@ -100,6 +100,18 @@ disk/container interface
 .It Pa /boot/kernel/aac.ko
 aac loadable module
 .El
+.Sh TUNING
+The read-only sysctl 
+.Va hw.aac.iosize_max
+defautls to 65536 and may be set at boot time to another value via
+.Xr loader 8 .
+This value determines the maximum data transfer size allowed
+to/from an array.  Setting it higher will result in better performance,
+especially for large sequential access patterns.  BEWARE: internal limitations
+of the card limit this value to 64K for arrays with many members.  While it
+may be safe to raise this value, do so AT YOUR OWN RISK.  Note also that
+performance peaks at a value of 96K, and drops off dramatically at 128K, due
+to other limitations of the card.
 .Sh DIAGNOSTICS
 Compiling with
 .Dv AAC_DEBUG