[DRBD-user] Differences between meta-disk and flexible-meta-disk?

Florian Haas florian.haas at linbit.com
Mon Jun 23 10:51:58 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.

> Hi,
> With all the discussions about external/internal metadata since a few
> days ago, I throw an eye to my drbd configuration.
> Then I wondered why I used flexible-meta-data internal in my
> configuration.
> So I looked to the drbd user guide:
> http://www.drbd.org/users-guide/re-drbdconf.html
> And now I'm more confused than two days ago :-)
> So, what is the real difference between flexible-meta-disk and meta-disk
> (internal or external) ?

flexible-meta-disk means that DRBD does not allocate a fixed metadata
area of 128M, but one that is roughly proportional to the DRBD device
size (_up_ to 128M for a 4TB device).

meta-disk means that DRBD does allocate a fixed metadata area of 128M,
regardless of DRBD device size.

Now brace yourself, internal metadata is _always_ flexible, "meta-disk
internal" and "flexible-meta-disk internal" are equivalent.

> And the cherry on the cake question is the following one: with external
> meta-data, what happens if you loose the device holding the meta-data?
> Are your data at risk, or does drbs can rebuild the meta-data (from the
> data or peer) ?

Neither. DRBD metadata is both per-device and per-node (one set of
metadata for each device on each node). So if you lose your metadata,
you must do "drbdadm down", replace the metadata device, then do
"drbdadm create-md" anew, and bring your device up again. This
inevitably necessitates a full sync from the other node (with
newly-created metadata, DRBD has no way of knowing which blocks are in
sync and which aren't), which will be initiated automagically upon


: Florian G. Haas
: LINBIT Information Technologies GmbH
: Vivenotgasse 48, A-1120 Vienna, Austria

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