Hi Jon,
You seem to be right that there isn't much written down about how snapshots
work under the bonnet in the LVM context, but the same ideas have been used
for other snapshot implementations for years. You could try searching for
filesystem snapshot technology in general, rather than anything
LVM-specific. For example, this article explains some of the basic concepts:
http://www.ibm.com/developerworks/tivoli/library/t-snaptsm1/index.html
LVM uses the copy-on-write method (this is what "COW" stands for).
On Thu, 10 Sep 2009, jonr@destar.net wrote:
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?
Because the snapshot only stores copies of the disk blocks that have been
modified on the original volume since the snapshot was taken (the blocks
that would otherwise be overwritten if you hadn't created the snapshot).
When you read from the snapshot, LVM uses never-modified blocks from the
original volume and pre-modification copies of blocks from the snapshot as
needed.
If the original volume became corrupt, the snapshot would also be unusable.
2. Can I create a DomU and then snapshot the LV and use the snapshot to
create other DomU's?
Yes - there are articles around about how to do this. If you intend to run
the DomU's on the same network, you should boot the copies into single-user
mode initially to modify their network identities so that they don't
conflict with each other. (Maybe you can find some other way of doing this
without booting the DomU's: I know that you can do this with VMWare, at
least for Linux guest OS'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
You can continue use the snapshot in principle, although you might like to
make the snapshot the same size as the original volume in that case to make
sure that it never fills up. In your example, when the accumulated changes
on the original volume since the snapshot was taken reach 10Gb, the snapshot
will be full and go invalid and you won't be able to use it.
If your aim is to have a single starting point to create more than one DomU,
you should copy the data out and keep it safe somewhere. You can then use
that copy to create further copies as needed. Personally, I would only boot
up the first copy (or the snapshot) to keep the OS up to date (which will
save you having to update every copy that you make from it).
4. Does the 0.11% mean that the 10GB snapshot file is only using .11% of the
10GB LV?
Yes, that's right.
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,
Have fun,
Peter.
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