From b93ce255b555d9b6c38c28adbc2f6d345b7a04a3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ulrich=20Sp=C3=B6rlein?= <uqs@FreeBSD.org>
Date: Thu, 5 Jan 2012 21:36:20 +0000
Subject: [PATCH] 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 | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/sbin/recoverdisk/recoverdisk.1 b/sbin/recoverdisk/recoverdisk.1
index 9eff8f747bb5..044b8017c8fe 100644
--- a/sbin/recoverdisk/recoverdisk.1
+++ b/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.