[DRBD-user] Problem with drbd and data alignment

Jan Schermer jan at schermer.cz
Mon Mar 27 15:13:40 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.


lsblk doesn't only show partitions, but other info as well.
e.g.
sdd                   8:48   0   477G  0 disk
└─zbackup_crypt     253:4    0   477G  0 crypt

that is not a partition, it just shows what the device is used for
it will be the same in your case.

Jan

> On 27 Mar 2017, at 15:05, Marco Marino <marino.mrc at gmail.com> wrote:
> 
> 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 <mailto: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 <tel:%2B43%201%20817%2082%2092%200>
> robert.altnoeder at linbit.com <mailto:robert.altnoeder at linbit.com>
> 
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