Shared LVM (thick/thin) on multiple hosts via ISCSI w/o CLVM/HA-LVM

[virtRoo]

Hi guys,

I've come across some cases where people simply expose LVM across multiple hosts via iSCSI without some form of proper clustering like CLVM or HA-LVM. I was wondering how reliable this would be?

We've actually dealt with a paid-for cloud management platform before that sets up LVM as VM virtual disks like this. ie. an iSCSI LUN is presented to multiple hypervisors, create a VG on top of the LUN from one of the hosts, and then create multiple LVs to be used as a VM disk. However, our experience with this kind of setup was never great, maybe we overlooked something.

Also, if an LVM is marked as 'shared' in Proxmox, does it mean Promox will modify any settings in /etc/lvm/lvm.conf on any hosts that have access to the LVM?

I also realised LVM thin pool cannot be shared as per Proxmox's user guide, is there a specific reason for this?

Thanks in advance.

 

[dcsapak]

Also, if an LVM is marked as 'shared' in Proxmox, does it mean Promox will modify any settings in /etc/lvm/lvm.conf on any hosts that have access to the LVM?

no, it simply states that the vg is shared among hosts (iow. all hosts have the same view of the vg)

I also realised LVM thin pool cannot be shared as per Proxmox's user guide, is there a specific reason for this?

yes, because thinlvm allocates extents on the fly when needed, and there is no mechanism to prevent multiple hosts from allocating the same blocks
lvm (not-thin), defines which blocks belong to which lv on lv creation, and as long the creation/deletion/modification of such lvs are guarded and locked, it is not a problem