Document the detrimental effect of kern.cam.XX.retry_count for recoverdisk(1)

recoverdisk(1) wants to know about read errors when doing the 1MB/64k
reads as fast as possible so it can continue and retry them later with
smaller reads (down to sector size). Any retries by lower layers will
only slow it down.

Prodded by:	arundel

sbin/recoverdisk/recoverdisk.1

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd May 1, 2011
+.Dd Jan 5, 2012
 .Dt RECOVERDISK 1
 .Os
 .Sh NAME
@@ -124,7 +124,11 @@ recoverdisk -b 0 /dev/ad3 /somewhere
 
 .Ed
 .Sh SEE ALSO
-.Xr dd 1
+.Xr dd 1 ,
+.Xr ada 4,
+.Xr cam 4,
+.Xr cd 4,
+.Xr da 4
 .Sh HISTORY
 The
 .Nm
@@ -149,6 +153,18 @@ flying by.
 This is harmless and can be avoided by setting
 .Fl b
 to no more than 128kB.
+.Pp
+.Nm
+needs to know about read errors as fast as possible, i.e. retries by lower
+layers will usually slow down the operation.
+When using
+.Xr cam 4
+attached drives, you may want to set kern.cam.XX.retry_count to zero, e.g.:
+.Bd -literal
+# sysctl kern.cam.ada.retry_count=0
+# sysctl kern.cam.cd.retry_count=0
+# sysctl kern.cam.da.retry_count=0
+.Ed
 .\".Pp
 .\"When reading from optical media, a bug in the GEOM framework will
 .\"prevent it from seeing that the media has been removed.