3 files changed, 26 insertions, 17 deletions

diff --git a/share/man/man4/aac.4 b/share/man/man4/aac.4
index 3988c4d751e7..22167b940017 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
diff --git a/sys/dev/aac/aac.c b/sys/dev/aac/aac.c
index 0ea8336ade22..75f01358841a 100644
--- a/sys/dev/aac/aac.c
+++ b/sys/dev/aac/aac.c
@@ -40,6 +40,7 @@
 #include <sys/systm.h>
 #include <sys/malloc.h>
 #include <sys/kernel.h>
+#include <sys/sysctl.h>
 
 #include <dev/aac/aac_compat.h>
 
@@ -183,6 +184,9 @@ static struct cdevsw aac_cdevsw = {
     0,			/* flags */
 };
 
+/* sysctl node */
+SYSCTL_NODE(_hw, OID_AUTO, aac, CTLFLAG_RD, 0, "AAC driver parameters");
+
 /******************************************************************************
  ******************************************************************************
 				Device Interface
diff --git a/sys/dev/aac/aac_disk.c b/sys/dev/aac/aac_disk.c
index a08789f893b4..d8331ea592c4 100644
--- a/sys/dev/aac/aac_disk.c
+++ b/sys/dev/aac/aac_disk.c
@@ -34,6 +34,7 @@
 #include <sys/param.h>
 #include <sys/systm.h>
 #include <sys/kernel.h>
+#include <sys/sysctl.h>
 
 #include <dev/aac/aac_compat.h>
 #include <sys/bus.h>
@@ -101,6 +102,14 @@ static driver_t aac_disk_driver = {
 
 DRIVER_MODULE(aacd, aac, aac_disk_driver, aac_disk_devclass, 0, 0);
 
+/* sysctl tunables */
+static unsigned int aac_iosize_max = 65536;	/* due to limits of the card */
+TUNABLE_INT("hw.aac.iosize_max", &aac_iosize_max);
+
+SYSCTL_DECL(_hw_aac);
+SYSCTL_UINT(_hw_aac, OID_AUTO, iosize_max, CTLFLAG_RD, &aac_iosize_max, 0,
+	   "Max I/O size per transfer to an array");
+
 /******************************************************************************
  * Handle open from generic layer.
  *
@@ -215,8 +224,6 @@ static int
 aac_disk_attach(device_t dev)
 {
     struct aac_disk	*sc = (struct aac_disk *)device_get_softc(dev);
-    int			sgspace;
-    int			maxsg;
     
     debug_called(4);
 
@@ -259,21 +266,7 @@ aac_disk_attach(device_t dev)
     disks_registered++;
 #endif
 
-    /*
-     * We can calculate the maximum number of s/g entries based on the size of
-     * the FIB and the command structures packed within it.
-     */
-    sgspace = (sizeof(struct aac_fib) - sizeof(struct aac_fib_header) - 
-	       imax(sizeof(struct aac_blockwrite),
-	       sizeof(struct aac_blockread)));
-    maxsg = (sgspace - sizeof(struct aac_sg_table)) /
-	     sizeof(struct aac_sg_entry);
-
-    /*      
-     * set the maximum I/O size to the theoretical worst maximum allowed by the
-     * S/G list size
-     */
-    sc->ad_dev_t->si_iosize_max = (maxsg - 1) * PAGE_SIZE;
+    sc->ad_dev_t->si_iosize_max = aac_iosize_max;
 
     return (0);
 }