[DRBD-user] Number of concurrent resources

Matthias Hensler lists-drbd at wspse.de
Mon Feb 24 14:34:08 CET 2014

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


On Sun, Feb 23, 2014 at 11:52:02PM +0000, Simon Ironside wrote:
> On 23/02/14 17:30, Ben RUBSON wrote:
> >>The question is, up to how many concurrent DRBD resources can be running ?
> A lot. From man drbd.conf (my version is 8.3.15):
> minor-count count
>     may be a number from 1 to 255.

I would like to dig a bit deeper here in how this affects DRBD 8.4.

The documentation states, that the maximum number of configurable DRBD
devices was increased to 2^20 (see A.5.5 at

However, I do not understand how this actually should work. Specifing a
minor_count of >255 in the configuration is not allowed. If I try to set
this to 355 I get an error message like this:
drbd.d/global_common.conf:18: minor-count 355 => 355 out of range [1..255].

Also it is not possible to load the drbd-module with a minor-count
exceeding 255.

Looking at the sourcecode (specifically at drbd_limits.h) it defines
DRBD_MINOR_COUNT_MAX as 255 (this is from current git of 8.4.4-6).

Strangely enough if I specify a minor_count of 255 in the
global_common.conf and bring up the module, it only initializes with the
default value of 32:
[root at leela drbd.d]# cat /sys/module/drbd/parameters/minor_count 
[root at leela drbd.d]# 

However, even if it only reports "32" I am still able to create a
resource pointing to /dev/drbd99. (This is not possible on DRBD 8.3, but
seems to be with 8.4).

But: trying to create a resource pointing to /dev/drbd299 is not
possible and bails out with:
The highest minor you have in your config is 299but a minor_count of 255 in your config!

I am not sure how the new limit of 2^20 should actually work, since DRBD
seems to use 147 as fixed majornumber and so only 255 minors would be

So is there any way with DRBD 8.4 to use more than 255 DRBD-devices? If
so I would be grateful for a pointer to the documentation. Thanks.

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