Explain more about the interleave factor. Mention mirroring, and add

a reminder that you can't mount half of a mirrored two-disk ccd.

1 files changed, 47 insertions, 13 deletions

diff --git a/share/man/man4/ccd.4 b/share/man/man4/ccd.4
index c581bf4..7f31d71 100644
--- a/share/man/man4/ccd.4
+++ b/share/man/man4/ccd.4
@@ -74,31 +74,64 @@ may be configured.
 .Pp
 A
 .Nm ccd
-may be either serially concatenated or interleaved.  If a
+may be either serially concatenated or interleaved.  To serially
+concatenate the partitions, specify the interleave factor of 0.
+.Pp
+There is a run-time utility that is used for configuring
+.Nm ccds .
+See
+.Xr ccdconfig 8
+for more information.
+.Ss The Interleave Factor
+If a
 .Nm ccd
 is interleaved correctly, a
 .Dq striping
-effect is achieved, which can increase performance.  The optimum interleave
-factor is typically the size of a track.  Since the interleave factor
-is expressed in units of DEV_BSIZE, one must account for sector sizes
-other than DEV_BSIZE in order to calculate the correct interleave.
-The kernel will not allow an interleave factor less than the size
-of the largest component sector divided by DEV_BSIZE.
+effect is achieved, which can increase sequential read/write
+performance.  The interleave factor is expressed in units of DEV_BSIZE
+(usually 512 bytes).  For large writes, the optimum interleave factor
+is typically the size of a track, while for large reads, it is about a
+quarter of a track. (Note that this changes greatly depending on the
+number and speed of disks.)  For instance, with eight 7,200 RPM drives
+on two Fast-Wide SCSI buses, this translates to about 128 for writes
+and 32 for reads.
 .Pp
-Note that best performance is achieved if all component disks have the same
+The best performance is achieved if all component disks have the same
 geometry and size.  Optimum striping cannot occur with different
 disk types.
 .Pp
-There is a run-time utility that is used for configuring
-.Nm ccds .
-See
+For random-access oriented workloads, such as news servers, a larger
+interleave factor (e.g., 65,536) is more desirable.  Note that there
+isn't much
+.Nm ccd
+can do to speed up applications that are seek-time limited.  Larger
+interleave factors will at least reduce the chance of having to seek
+two disk-heads to read one directory or a file.
+.Ss Disk Mirroring
+You can configure the
+.Nm ccd
+to
+.Dq mirror
+any even number of disks.  See
 .Xr ccdconfig 8
-for more information.
+for how to specify the necessary flags.  In an event of a disk
+failure, you can use
+.Xr dd 1
+to recover the failed disk.
+.Pp
+Note that a one-disk
+.Nm ccd
+is not the same as the original partition.  In particular, this means
+if you have a filesystem on a two-disk mirrored
+.Nm ccd
+and one of the disks fail, you cannot mount and use the remaining
+partition as itself; you have to configure it as a one-disk
+.Nm ccd.
 .Sh WARNINGS
 If just one (or more) of the disks in a
 .Nm ccd
 fails, the entire
-file system will be lost.
+file system will be lost unless you are mirroring the disks.
 .Sh FILES
 /dev/{,r}ccd* - ccd device special files.
 .Pp
@@ -106,6 +139,7 @@ file system will be lost.
 The concatenated disk driver was originally written at the University of
 Utah.
 .Sh SEE ALSO
+.Xr dd 1 ,
 .Xr MAKEDEV 8 ,
 .Xr ccdconfig 8 ,
 .Xr config 8 ,