Update the cd(4) man page for the CAM cd driver.
This includes a description of the changer timeout kernel options and sysctl variables. I didn't check to make sure the ioctl descriptions are up to date; that will come sometime later. (The ioctls haven't changed in the CAM driver, but I'm not sure if the man page was in sync with even the old driver.)
This commit is contained in:
Kenneth D. Merry
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dfce019974
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2d8eb2c034
@ -23,9 +23,9 @@
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $Id: cd.4,v 1.10 1997/02/22 13:24:23 peter Exp $
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.\" $Id: cd.4,v 1.11 1997/03/21 20:13:23 mpp Exp $
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.\"
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.Dd January 16, 1996
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.Dd October 10, 1998
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.Dt CD 4
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.Os FreeBSD
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.Sh NAME
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@ -34,6 +34,8 @@
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.Sh SYNOPSIS
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.Cd device cd
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.Cd device cd1 at scbus0 target 4 lun 0
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.Cd options "CHANGER_MIN_BUSY_SECONDS=3"
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.Cd options "CHANGER_MAX_BUSY_SECONDS=11"
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.Sh DESCRIPTION
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The
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.Nm cd
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@ -53,14 +55,14 @@ is unmounted.
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In general the interfaces are similar to those described by
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.Xr wd 4
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and
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.Xr sd 4 .
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.Xr da 4 .
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.Pp
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As the
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.Tn SCSI
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adapter is probed during boot, the
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.Tn SCSI
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bus is scanned for devices. Any devices found which answer as `Read-only'
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type devices will be `attached' to the
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bus is scanned for devices. Any devices found which answer as CDROM
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(type 5) or WORM (type 4) type devices will be `attached' to the
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.Nm
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driver.
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Prior to
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@ -351,13 +353,6 @@ struct ioc_pitch
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};
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.Ed
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.El
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.Pp
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In addition the general
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.Xr scsi 4
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ioctls may be used with the
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.Nm
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driver, if used against the `whole disk' partition (i.e.
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.Pa /dev/rcd0c ) .
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.Sh NOTES
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When a
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.Tn CD-ROM
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@ -383,6 +378,51 @@ drives for which audio will not work. Some work is planned to support
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some of the more common `broken'
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.Tn CD-ROM
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drives; however, this is not yet under way.
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.Sh CHANGER OPERATION
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This driver has built-in support for LUN-based CD changers. A LUN-based CD
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changer is a drive that can hold two or more CDs, but only has one CD
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player mechanism. Each CD in the drive shows up as a seperate logical unit
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on the
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.Tn SCSI
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bus. The
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.Nm cd
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driver automatically recognizes LUN-based changers, and routes commands for
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changers through an internal scheduler. The scheduler prevents changer
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"thrashing", which is caused by sending commands to different LUNs in the
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changer at the same time.
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.Pp
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The scheduler honors minimum and maximum time
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quanta that the driver will spend on a particular LUN. The minimum time
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is the guaranteed minimum amount of time that the driver will spend on a
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given LUN, even if there is no oustanding I/O for that LUN. The maximum
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time is the maximum amount of time the changer will spend on a LUN if there
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is oustdanding I/O for another LUN. If there is no outstanding I/O for
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another LUN, the driver will allow indefinite access to a given LUN.
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.Pp
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The minimum and maximum time quanta are configurable via kernel options and
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also via sysctl variables. The kernel options are:
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.Bl -tag -width 1234 -compact
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.It Cd options "CHANGER_MIN_BUSY_SECONDS=3"
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.It Cd options "CHANGER_MAX_BUSY_SECONDS=11"
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.El
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.Pp
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The sysctl variables are:
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.Pp
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.Bl -tag -width 1234 -compact
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.It Cd kern.cam.cd.changer.min_busy_seconds
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.It Cd kern.cam.cd.changer.max_busy_seconds
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.El
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.Pp
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It is suggested that the user try experimenting with the minimum and
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maximum timeouts via the sysctl variables to arrive at the proper values
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for your changer. Once you have settled on the proper timeouts for your
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changer, you can then put them in your kernel config file.
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.Pp
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If your system does have a LUN-based changer, you may notice that the
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probe messages for the various LUNs of the changer will continue to appear
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while the boot process is going on. This is normal, and is caused by the
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changer scheduling code.
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.Sh FILES
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.Bl -tag -width /dev/rcd[0-9][a-h] -compact
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.It Pa /dev/cd[0-9][a-h]
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@ -398,7 +438,7 @@ devices
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None.
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.Sh SEE ALSO
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.Xr scsi 4 ,
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.Xr sd 4 ,
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.Xr da 4 ,
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.Xr disklabel 5 ,
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.Xr disklabel 8 ,
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.Xr cd 9
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@ -409,7 +449,28 @@ were poorly chosen, and a number of spelling errors have survived in
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the names of the
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.Fn ioctl
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commands.
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.Pp
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There is no mechanism currently to set different minimum and maximum
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timeouts for different CD changers; the timeout values set by the kernel
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options or the sysctl variables apply to all LUN-based CD changers in the
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system. It is possible to implement such support, but the sysctl
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impelmentation at least would be rather inelegant, because of the current
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inability of the sysctl code to handle the addition of nodes after compile
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time. Thus, it would take one dynamically sized sysctl variable and a
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userland utility to get/set the timeout values. Implementation of separate
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timeouts for different CD devices in the kernel config file would likely
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require modification of
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.Xr config 8
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to support the two timeouts when hardwiring
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.Nm cd
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devices.
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.Sh HISTORY
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This
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.Nm
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driver appeared in 386BSD 0.1.
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.Nm cd
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driver is based upon the
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.Nm cd
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driver written by Julian Ellischer, which appeared in 386BSD 0.1. The
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CAM version of the
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.Nm cd
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driver was written by Kenneth Merry and first appeared in
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.Fx 3.0 .
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