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Re: [linux-lvm] Snapshots and disk re-use

On 06/04/2011 02:20, James Hawtin wrote:
On 06/04/2011 00:50, Jonathan Tripathy wrote:

If the PV used for snapshots were to fail while the snapshot was open, or the server rebooted and the PV wasn't available at boot, what would happen? I ask these questions because a loopback device or iSCSI is really my only feasible option right now for the snapshot PV...

What would happen is... if the file systems are mounted at boot time (in the fstab) it will fail the fsck because the device is not there. and drop to single user mode, you could then edit the fstab to to remove those file systems that would bring the system online, at which point you could fix what stopped the iscsi from working, and mount the file systems.

At one place I worked they never mounted the data file systems at "boot" but in a rc.local so the system always got to be interactive before any problems so it was easy to go in and fix.
Since my vg is used purly for virtualisation, I don't mount any LVs in fstab in my host OS. LVs only get mounted once VMs start up.

DO NOT... create a loop back device on the same on a file system that the loopback that will then form a pv of, if you do your system is DOOOMMMED! to get it to boot again your have to mount a part volume group and block copy the devices to a new one, even worse if you extended the file system with the loop back on it onto the pv of the loop back it will NEVER work again. So the only place you can create a loopback device is outside of a vg it is to be a part of and frankley better that its in NO vg as you may get recursion problems.
Oh no, wouldn't dare to do that :) I was thinking of creating the image file on a non-LVM area of the drive (somewhere in the host OS partition)

The problem with a loop back is that you need to do a the loopback setup to enable the device before the vgscan and vgchange can bring it online in the volume, very hard to get right at boot time. If you have partitioned it you will also need to do kpartx.
Well I wouldn't partition the image file, seems more trouble than it's worth. I did find an article from Novell that seems to guide me through what to do if my machine were to reboot with a PV missing:


If you use loopbacks i would extend the volume group onto the disk only during backups then nreduce it out afterwards to reduce risks.
Good Idea.

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