[DRBD-user] Problem with drbd and data alignment

Marco Marino marino.mrc at gmail.com
Mon Mar 27 15:05:41 CEST 2017

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


Hi Robert,
I think the problem is related to the fact that there is a (LVM) partition
on top of the drbd device. I tried different configurations and if I use a
raw device for the drbd resource and then I use the drbd device without
partitions as a PV the problem disappear. Anyway, I dont know if this is
really a problem because there is not overlap at all. I (double) checked
this using fdisk on /dev/sde and on /dev/drbd2. Both show me a partition
that starts at sector 2048. The only problem is that when I use lsblk I see
that inside /dev/sde there are 2 devices: /dev/sde1 and /dev/drbd2. I'd
like to know if this can cause some kind of problem with performance or
data corruption....
Probably the question is: can I use a raw device as PV? As suggested here
<https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Logical_Volume_Manager_Administration/LVM_components.html#multiple_partitions>
it is possible but there is some problem with the management if I need to
enlarge the volume but I don't need this.

Marco




2017-03-27 9:54 GMT+02:00 Robert Altnoeder <robert.altnoeder at linbit.com>:

> The system looks for partition information at a certain offset on a
> storage device. If DRBD is used to write directly to a raw storage
> device, the effect of partitioning the DRBD device on the backing device
> is the same as if the backing device had been partitioned directly with
> no DRBD involved.
> Therefore, the system will see the partition information on the backing
> device.
>
> DRBD internal meta data is written to the end of the backing device
> (without using any partitioning). The space occupied by the internal
> meta data will be missing from the DRBD device's size (the DRBD device's
> size will be the backing device's size minus the space reserved for
> internal meta data).
>
> On 03/25/2017 02:57 PM, Yannis Milios wrote:
> >
> > Not 100% sure but the first 'partition' could be where drbd stores
> > metadata?
> >
> > Your setup should be fine as long ad you can deal in an easy manner
> > with future resizing of the backing device (sde).
> >
> > Yannis
> >
> > root at iscsi2 ~]# lsblk
> > NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
> > ........
> > sde 8:64 0 3,7T 0 disk
> > ├─sde1 8:65 0 3,7T 0 part
> > └─drbd2 147:2 0 3,7T 0 disk
> > --
> > Sent from Gmail Mobile
> >
> br,
> --
> Robert Altnoeder
> +43 1 817 82 92 0
> robert.altnoeder at linbit.com
>
> LINBIT | Keeping The Digital World Running
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