[DRBD-user] DRBD8, dedicated repository for PVE 4

Jasmin J. jasmin at anw.at
Sat Dec 24 05:03:10 CET 2016

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

Hello Cesar!

 > I am using only LVM on top of DRBD because i think this:
 > 1- Using "LVM / DRBD8 / Raw VM disk",  i have less layers in the middle,
 > then, the I/O will be faster.
 > Is it correct?, or will be minimal the difference of performance?
I guess there is no big difference between
    PhysDisk / DRBD8 / LVM
    PhysDisk / LVM / DRBD8
I read LVM is really simple implemented within the Kernel via a mapper and
doesn't add much execution time. And the additional LVM execution time is
the same for both variants.

When you are using PhysDisk / DRBD8 / LVM in Proxmox you can define this
as a shared Storage, but who act/deactivates DRBD8 in this case?
I had it hat way, but then I discovered I needed other scripts to deal with
DRBD8. Moreover I had multiple VMs using the same DRBD8 device because i had
a single LVM VG for all my VMs disks. This was a big problem, because I had
to keep all VMs using the same DRBD8 disk on one server. Otherwise I could
get a Split Brain situation and also I needed dual primary mode in DRBD8.

So I have now PhysDisk / LVM / DRBD8 and my Proxmox Storage Plugin. I
configure LVM and DRBD8 completely manual (per diks!). Then Proxmox and my
Plugin do the magic of act/deactivating and primary/secondary switching. And
the most important, I have now independent DRBD8 instances per VM disk. Using
this configuration with single primary mode is very save, I guess and should
avoid a split brain situation per design. This was not possible in Proxmox
prior, because of the missing DRBD8 Storage Plugin.

 > 2- As i use the DRBD verify online once a week,  and i want that this
 > process not take long, then, my LVM partitions on top of DRBD only has the
 > minimal size necessary for the disks of VMs, instead of use the complete
 > disk of the server.
I had it configured exactly the same, but then you share a DRBD8 for several
VMs. If you have a DRBD8 / LVM instance per VM you should be fine, but this
requires scripts to activate DRBD8 and you have to deal in this script with
VMs moved to the other server and thus are not allowed to switch DRBD8 to
primary (unless you are using dual primary mode).

 > The disadvantage of this strategy is that when i need resize the disk of the
 > VM, i must do it in offline mode, but with a few steps. I can live well with
 > this method.
I am not sure if PhysDisk / LVM / DRBD8 allows online resizing. I never tried
it. But when I think now ... LVM allows simply resizing the LV. DRBD8 allows
this too: https://www.drbd.org/en/doc/users-guide-84/s-resizing
After resizing LVM/DRBD8 log into your VM and do an online resize of the
mounted partition (sudo resize2fs /dev/vd*, if it is ext4).


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