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





On 06/04/2011 01:42, James Hawtin wrote:
On 06/04/2011 00:28, Jonathan Tripathy wrote:
Ok, i think I get it now. At the minute, my vg (vg0) only has on PV in it (/dev/md3 which you can tell is a mdadm RAID device). I wasn't aware you could add more PVs (that's pretty cool!). So, let's say I had a spare partition (/dev/hdb7 as an example). To my vg0 volume group, I would firstly:

pvcreate /dev/hdb7
vgextend /dev/hdb7

Right however danger warnings are going off in my mind now!

Then, every time I create a new customer LV, I would do:

lvcreate -nNewCustomerLV -L20G vg0 /dev/md3

yes that would work


Then, every time I wanted to create a snapshot:

lvcreate -L20G -s -n data_snap /dev/vg0/NewCustomerLV /dev/hdb7


Yes

Is that correct? No Leakage? And no zeroing needed?
Indeed


Side note: Since I didn't partition my servers with this in mind, my new PV will probably have to be an iSCSI device located on a remote target :( Either that or use a loopback device with an image, but I'd be scared that the system would not boot properly. Can you give me any tips on how to use an image file as a PV just for snapshots?

Ok, there has been alot of dangerous talk here, i assume you are using an md device so you can mirror things.
Correct
If you added a single disk to that, and that disk failed you would have a major problem. Likewise if you rebooted with open snaps and iscsi you would need that iscsi device available during computer boot. I REALLY hope you do not have your local FS on the same vg as your data. As this would result in a non booting machine.
Nope. My root partition is not on LVM at all, but just on a regular md partition. The LVM LVs are used for virtual machine hosting for customers. Also, I used just /dev/hdb7 as an example, but it does bring up some interesting questions:

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...


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