[DRBD-user] Online resizing DRBD resources with external metadata

Drew Hastings dhastings at crucialwebhost.com
Wed Mar 28 02:08:51 CEST 2018

In 8.4 the documentation for this makes it look rather simple :

It mentions, "Online shrinking is only supported with external metadata." .

Using external metadata, you would be expected to size the resource's
backing device and the external metadata appropriately. The size of that
metadata device will be based off of the size of the resource’s backing
device. So the metadata device size is roughly proportional to
the resource’s backing device size. With that in mind, the documentation
makes no mention of resizing the underlying metadata device in the process
of resizing the resource’s backing device.

When increasing the size of the DRBD resource, presumably you would be
expected to increase the size of the external metadata device. That's fine
- but does DRBD actually handle the fact that you've done that? When you
run "drbdadm resize", does that look at the newly sized metadata device? My
concern is that if for instance you were to go from a 100MiB resource with
a 4MiB external metadata device, then scale the 100MiB to 1TiB... you'd
need to increase the metadata device size. The documentation doesn't make
much mention of that aspect of this.

Reducing the size of the resource (and metadata) seems even less clear. Can
you shrink the DRBD resource then subsequently reclaim the metadata device
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