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[linux-lvm] Snapshots



Hello List,

I don't get it!

I have a xen system that has a DomU of FC11 with a 20GB LV for the disk. I created a snapshot using this command:

lvcreate -L +10G -s -n s_cgate-be1 /dev/xenvg/cgate-be1

This is the output of an 'lvdisplay' of the snapshot LV:

lvdisplay /dev/xenvg/s_cgate-be1
  --- Logical volume ---
  LV Name                /dev/xenvg/s_cgate-be1
  VG Name                xenvg
  LV UUID                7c1kEs-xYoO-Qxem-2E2M-hlee-pc4Q-zm6TZ5
  LV Write Access        read/write
  LV snapshot status     active destination for /dev/xenvg/cgate-be1
  LV Status              available
  # open                 2
  LV Size                20.00 GB
  Current LE             5120
  COW-table size         10.00 GB
  COW-table LE           2560
  Allocated to snapshot  0.11%
  Snapshot chunk size    4.00 KB
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:15

I then shutdown the DomU and pointed the 'Disk...' line in the config file for the DomU to the snapshot. It boots.

OK, I must be missing the simple answer because I cannot seem to grasp the concept of snapshots.


1. How can I have a 20GB LV as a disk and the snapshot be 10GB and boot the entire OS?

2. Can I create a DomU and then snapshot the LV and use the snapshot to create other DomU's?

3. If 2 is yes, would I want to continue using the snapshot as the disk or is there something else that should be done, i.e. dd the drive to a new LV?

In the 'lvdisplay' of the snapshot I have these extra lines:
  LV snapshot status     active destination for /dev/xenvg/cgate-be1
  # open                 2
  COW-table size         10.00 GB
  COW-table LE           2560
  Allocated to snapshot  0.11%
  Snapshot chunk size    4.00 KB

4. Does the 0.11% mean that the 10GB snapshot file is only using .11% of the 10GB LV?


If there is a good doc out there explaining all of this could someone just post a link. I don't mind reading but am not getting the answers to the above. I have found lots of docs explaining how to do it but nothing going much further than that.

Thanks for any help,

Jon



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