[DRBD-user] DRBD-LVM sandwich questions

Adam Wilbraham adam.wilbraham at technophobia.com
Wed Jul 22 12:07:32 CEST 2009

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

Hi Dmitry,

I'm using a setup very similar to yours apart from the last part - I use 
a 1 LV to Xen partition mapping for all of our PV domUs. Its only the 
HVM ones which I use as a full disk image, but anyway its not too relevant.

Our backup strategy currently involves taking daily LVM snapshots and 
rsyncing data from them (or backing up using dd in case of HVM). Doing 
this means you aren't doing anything to the underlying device so removes 
any risk - just make sure your snapshot is big enough to cope with the 
amount of data that will change whilst it exists.

Whenever I do a resize of a LV & subsequent DRBD device I always 
schedule a reboot of the domU afterwards to make sure that the OS 
properly picks up the resize. I've not had any problems with either of 
these approaches so far.

On 21/07/09 22:49, Dmitry S. Makovey wrote:
> Hi everybody,
> we're running LVM-DRBD-Xen-LVM sandwich on our boxes and got an interesting
> question:
> as you can see from above we have LV (say /dev/system/lv1) which we use as a
> base for DRBD device (say /dev/drbd0, named "vmspace") which is then used as
> a raw disk for Xen VM (say myVM). Inside of myVM we use LVM again to manage
> partitions etc. (say we have /dev/vmsystem VG inside with lots of various LVs
> in that VG).
> Now above we've used to be able to dynamically grow our VMs if needed.
> Scenario looks like this: we do
> lvresize /dev/system/lv1
> drbdadm resize vmspace
> ...inside of VM
> fdisk /dev/xvda
> ...add another partition /dev/xvda3
> vgextend /dev/xvda3
> lvresize /dev/vmsystem/lvX
> resize2fs  /dev/vmsystem/lvX
> Now another thing that happened after using above scenario:
> running fdisk -l /dev/system/lv1 we saw different picture on primary and
> secondary DRBD box until secondary was rebooted.
> Now the actual questions:
> 1. why did fdisk report /dev/system/lv1 layout different for primary and
> secondary (probably not-so-much DRBD question, but it kind of is related). By
> different I mean - it was showing the size of /dev/xvda correctly, but it was
> showing only 2 partitions on secondary box and 3 on primary
> 2. is it OK to use LVM snapshots for /dev/system/lv1 on secondary box to
> create "copies" of running VM to get some migration/testing done? (I realize
> problems about snapshoting running system, but are there any other problems)?
> 3. I remember seeing some information about "not touching raw devices
> underneath DRBD devices" and I also remember reading that even read-only
> mounts along with other apparently "innocent" operations still modify
> underlying devices. Does anybody have reference to the list of those "big
> no-no's" ?
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