[DRBD-user] iscsi + md0 = tell me why this is a bad idea

Lars Ellenberg lars.ellenberg at linbit.com
Wed Oct 22 10:27:57 CEST 2008

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, Oct 21, 2008 at 06:12:47PM -0500, Artur (eBoundHost) wrote:
> you said:
> =============================
> think about what can go wrong.
> and how to recover from that.
> what if ...
>   one target breaks
>   one link breaks
>   one initiator breaks
> how to
>   do a failover.
>   do a failback.
>   determine "better" (more recent) data.
>   determine resync direction.
>   determine which areas of the disks to sync.
>   determine data divergence.
> I think in all areas drbd does better than nbd/iscsi + md raid1,
> but I am happy to hear all ideas, and use them as inspiration
> for future linux storage replication solutions.
> =============================
> Well the good thing about that is that you never have to guess what  
> happened.  Since the md raid will lose one of the drives and still run  
> on the other one,

don't think just one specific failure scenario.
think them all through.

don't settle for "that can be solved".

you need one configuration to handle all of the failure scenarios.

> you never think about which one is current.

been there done that.
of course you have to think about which one is current.

> The one  that does not have a connection is the one that needs to be
> resynced.

right. and which one is that?
or do you think of only one head node (the one that runs the iscsi
initiators and puts and md over those)?
well, then what if that head node fails?

what if it first loses connectivity to one target, then crashes (because
of a multi-stage power failure or for whatever reason)?

> And you don't have to resync the entire drbd0 volume,

we don't.
we always go for "just the changed blocks".
that is what we have the bitmap and "activity log" for.

> just however much  you allocated through LVM to be exported.

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

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