Make CCD be able to read and write Linux software raids.

Supported for raid-0 with <n> disks, raid-1 with 2 disks.

Manpages have examples, warnings etc.

Test scripts on
http://www.cons.org/cracauer/ccdconfig-linux/
Reviewed by:	alfred

1 files changed, 28 insertions, 0 deletions

diff --git a/share/man/man4/ccd.4 b/share/man/man4/ccd.4
index 4ff4002..d49a82d 100644
--- a/share/man/man4/ccd.4
+++ b/share/man/man4/ccd.4
@@ -173,6 +173,34 @@ You cannot replace a disk in a mirrored
 .Nm
 partition without first backing up the partition, then replacing the disk,
 then restoring the partition.
+.Ss Linux compatibility
+The Linux compatibility mode does not try to read the label that Linux'
+md(4) driver leaves on the raw devices.  You will have to give the order
+of devices and the interleave factor on your own.  When in Linux
+compatibility mode, ccd will convert the interleave factor from Linux
+terminology.  That means you give the same interleave factor that you
+gave as chunk size in Linux.
+.Pp
+If you have a Linux md(4) device in "legacy" mode, do not use the
+CCD_LINUX flag in
+.Xr ccdconfig 8 .
+Use the CCD_NO_OFFSET flag instead.  In that case you have to convert
+the interleave factor on your own, usually it is Linux' chunk size
+multiplied by two.
+.Pp
+Using a Linux raid this way is potentially dangerous and can destroy
+the data in there.  Since FreeBSD does not read the label used by
+Linux, changes in Linux might invalidate the compatibility layer.
+.Pp
+However, using this is reasonably safe if you test the compatibility
+before mounting a raid read-write for the first time.  Just using
+ccdconfig without mounting does not write anything to the Linux raid.
+Then you do a fsck.ex2fs on the ccd device using the -n flag.  You can
+mount the filesystem readonly to check files in there.  If all this
+works, it is unlikely that there is a problem with ccd.  Keep in mind
+that even when the Linux compatibility mode in ccd is working
+correctly, bugs in FreeBSD's ex2fs implementation would still destroy
+your data.
 .Sh WARNINGS
 If just one (or more) of the disks in a
 .Nm