On Wed, 4 Jun 2008, Brian J. Murrell wrote:
On Wed, 2008-06-04 at 07:07 -0400, Mikulas Patocka wrote:
A pedantic example would be to create an origin, O1 and then a snapshot
of O1 called S1. Make a bunch of changes to S1 and then create another
snapshot of O1 called S2 and then block copy from S1 to S2 (presumably,
although I don't know for sure, S2 only contains the same changed blocks
as S1).
No, if you block copy over /dev/vg/snapshot device, it will contain all
the blocks (regardless if they differ from the origin or not).
So my premise of copying from S1 to S2 and the result that S2 would only
contain real blocks for what differs to the origin and pointers for
blocks that don't differ is false?
Yes, it's false.
If you overwrite a snapshot, you always allocate new exceptions of the
written blocks --- regardless if the blocks differ from the origin or not.
There could theoretically be compare function, testing if the write equals
to actual data and dropping the write eventually --- but it would be too
much coding overhead for too little practical advantage.
For certain use cases I think this would be a great advantage. Think
about this:
1. Create an LV and install a linux distro into it.
2. Make a snapshot and customize to the settings for a given
configuration (i.e. a group of hosts)
3. Now I want to make an (initially) identical (to the snapshot
customized in step 2) snapshot for each node in that
configuration
Being able to do:
# lvcreate -L5G -n pristine node_group
[ install linux distro into /dev/node_group/pristine ]
# lvcreate -s -L$size -n config1_master pristine
# for node in $nodelist; do
lvcreate -s -L$size -n $node pristine
dd if=/dev/node_group/config1_master of=/dev/node_group/$node
done
And have each of those snapshots be as efficient as config1_master.
You can do this if you copy the exception store (i.e. don't copy
/dev/vg/snap or /dev/mapper/vg-snap --- instead, copy
/dev/mapper/vg-snap-cow). Copy it to a new linear logical volume and then
use lvconvert -s to turn it into a snapshot. When you copy
/dev/mapper/vg-snap-cow, the snapshot and origin must be idle ---
otherwise you'd be copying data while it's being modified.
Mikulas
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