Re: [libvirt] [PATCH 1/2] logical: Use correct syntax for thin/sparse pool creation
On 12/16/2014 10:20 AM, Daniel P. Berrange wrote:
On Fri, Dec 12, 2014 at 04:14:29PM -0500, John Ferlan wrote:
<...snip...>
I'm unclear still on what the difference is between a thin snapshot (with
no backing volume) and a thin volume ?
I've asked that question of someone from lvm...
The thin shapshot has a 'hidden' backing pool of sorts (seen with lvs -a)
# lvs LVM_Test -a
...
test LVM_Test swi-a-s--- 4.00m [test_vorigin] 0.00
[test_vorigin] LVM_Test owi-a-s--- 8.00m
...
When the thin snapshot is removed, the [test_vorigin] is removed as well.
The target file created (/dev/LVM_Test/test) will have the 8M size as seen
via the virsh vol-list --details or virsh vol-info commands. Compare that
to the thin lv's which have the allocation size listed for capacity until patch 2
is added which asks the pool for it's size (e.g. the --virtualsize value).
FWIW, the original intent was that this provide a volume that is equivalent
semantically to a sparse file created on a filesystem. ie the LVM equivalent
to 'dd if=/dev/zero of=foo.img seek=1G count=0'
Given Jan's point/concern - I'll post a patch that adds the "--type
snapshot"
until more time/effort can be devoted to thin pool support.
John