[DRBD-user] "dd" meta-data when DRBD's role is Primary or Secondary

Lars Ellenberg lars.ellenberg at linbit.com
Tue Jul 27 16:38:01 CEST 2010

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

On Tue, Jul 27, 2010 at 11:06:42PM +0900, Junko IKEDA wrote:
> Hi,
> I'm trying to confirm whether DRBD + Heartbeat cluster can put up with
> the following scenario.
> (1) startup DRBD + Heartbeat
>      Active node's role: Primary
>      Standby node's role: Secondary
> (2) crash the meta-data
>      call "dd" command on the both of Active and Standby node like this;
>      They have internal meta-data on /dev/cciss/c0d0p6.
>      # dd if=/dev/zero of=/dev/cciss/c0d0p6 count=100000
> after that, nothing has happened.

assuming you have a partition table on sda, and you do
	grep sda /proc/partitions
	dd if=/dev/zero of=/dev/sda bs=4k count=1
	grep sda /proc/partitions
"nothing has happened" either,
but your next reboot won't get very far.

Assuming you have some decent ram, and run
	find /mntpoint -ls > /dev/null
to get a not too large file system dentry and inodes into the cache.
then you dd if=/dev/zero of=/underlying-block-device,
you will still be able to cd, and maybe even ls -l and stuff,
as long as it stays in the cache.

> Active node kept running its service,
> and after stopping Active node, Standby node took over the service.
> This behavior is not a problem,
> but I just want to know whether DRBD can keep its meta-data on
> memory(?) if DRBD's role is Primary or Secondary.

No. It knows about its _state_, and on state changes,
writes (part of) that state to stable storage.

> If I down the DRBD's resource and "dd" the meta-data,
> it seemed that meta-data was crashed, and I needed to recreate it.
> # drbdadm down all
> # dd if=/dev/zero of=/dev/cciss/c0d0p6 count=100000
> # drbdadm up all
>     ---> Failed!
> # drbdadm create-md all

So what?  What are you going to prove here?  That you are able to use dd
to damage your system?  No news in that.

Sorry, but this is ridiculous.
I really don't see where this should be going.

: Lars Ellenberg
: LINBIT | Your Way to High Availability
: DRBD/HA support and consulting http://www.linbit.com

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