RE: [SSI-devel] [DRBD] drbd-0.7.6-ssi_pre17 patch

[Roger T. <pe...@ho...>]

Hi En Chiang,

>1. The patch to linuxrc doesn't take into consideration the meta-device
being different from the ll-device. This would be useful to mirror an 
existing root
(can't resize mounted fs).

True I left it out.  It is up to the user to mknod the md-dev because it may 
take
some processing to figure out the major minor even if one issued a sh-md-dev 
to get
the path to their meta device.  On the other hand it is easy (relatively) to 
resize
an existing root by booting with a RH/FC rescue CD or Debian Knoppix CD.

However let me know if someone made a script to mknod the md-dev in linuxrc 
to
handle either ide and scsi devices on all platforms.  For the time it takes 
to
script that, I'd say just issue the one line mknod command for the md-dev 
after
patching linuxrc!  Later on if we do include DRBD linuxrc mods into mkinitrd 
and not
as a separate contrib package we might consider scripting that.

>
>2. /etc/init.d/drbd is changed to fsck and mount the drbd devices from
fstab. The initialisation could be moved into rc.sysinit and
>rc.sysinit.nodeup before the fsck/mount calls. Attached is a patch to
rc.sysinit.nodeup which I've been using. The drawback of this would be that 
the
/etc/init.d/drbd script would not be used at all.

I think it is a bad idea to replace the drbd script with the rc.sysinit* 
patches.
However it is a good idea to have the DRBD script deal with DRBD stuff only 
and let
the SSI-modified rc.sysinit scripts deal with fsck and mount.

1.  The user can no longer manually choose not to run DRBD during SysVinit 
which
might be something the user wants to do for non-root mounts.
2.  Your rc.sysinit needs to assure drbd is loaded or just skip all the drbd 
stuff.
3.  Change test to [ $1 -ne 0 ]; after grep Primary in case grep is broken.
4.  You need to include drbd global options from /etc/drbd.conf.

If we choose to do it this way a more elegant solution would be to call the 
original
/etc/init.d/drbd script rather than re-writing it in rc.sysinit.  Chkconfig 
drbd off.  Then deal with changing state to Primary.

# DRBD initializtion
[ -x /etc/init.d/drbd ] && /etc/init.d/drbd start
# For all DRBD resources change to Primary state

>
>3. Attached is a howto (based on the old drbd-Howto) which explains the
installation procedure. Do take a look and let me know what you think. I'll 
update
the non-root filesystem failover tomorrow.

1. Still need to s/0.7.5/0.7.6/g
2. No need to mkdir /dev/drbd and mknod /dev/drbd/0 since my linuxrc patch 
does that.
3. No need to mknod the root device (same reason).
4. Update the instructions to patch linuxrc with my linuxrc.patch.
5. On step 12 in grub.conf don't replace the UUID=... with /dev/drbd/0 (same 
reason).
6. Harsh but is a possibility and has happened with a non-stable SSI kernel. 
  Add note that DRBD destroys data integrity in the following situation:

Node 1 hangs/crashes.  Node 2 takes over, then Node 2 crashes before Node 1
recovers; Node 2 crashes before or during node 1 initrd CLMS discovery 
(root) or
drbd connection (non-root).  So Node 1 takes over.  Node 1 storage device fs
consistent, but contains old or missing data - node 2 storage has the latest 
data!
When node 2 recovers DRBD will detect split-brain and not sync, but next 
time drbd
connects node 2 will be sync target.  The damage is potentially extreme.


Regards,

Roger