Dne 14.1.2015 v 13:38 Bram Klein Gunnewiek napsal(a):
On 01/14/2015 12:42 PM, Zdenek Kabelac wrote:
Dne 14.1.2015 v 11:18 Bram Klein Gunnewiek napsal(a):
Thanks for your reply. I tested it out and it does not seem to work (lvm2
version used is 2.02.98-6ubuntu2):
yep - too old for this feature being supported.
Ok, then we have to do it ourself since we don't want to go with backports /
custom versions on our systems
lvcreate -n testvolume3 -L 1G vg
lvcreate -n testvolume3_snapshot -s /dev/vg/testvolume3 -l100%ORIGIN
dd if=/dev/zero of=/dev/vg/testvolume3 bs=1G count=1
After this the snapshot is invalid:
testvolume3_snapshot vg swi-I-s-- 1.00g testvolume3 100.00
I checked out the lvm2 source code and found the methods cow_max_extents and
_cow_max_size in snapshot_manip.c. Do these methods calculate the exact max
size of the snapshot meta data?
yes.
It's probably worth to note there has been some related kernel fixes for old
snaps - otherwise there has been theoretical path to create much bigger
snapshots then the origin size was - though it's nearly impossible to reach
such state.
Thanks again. I did some tests and calculations and it seems that the overhead
is ~0.39% of the origin size when chunk sizes of 4KiB are used. Bigger chunk
size means less overhead. If we use 0.5% of the origin space as meta data
overhead we should be on the safe side I think?
It cannot be defined as some 'percentage' value - as there are even
combinations of LV sizes and extent size where the percentage might be over
50% (thought those are not really usable in real life).
lvm2 code is the only valid calculation how to estimate full max size of
snapshot for various combinations of those parameters.
Zdenek
Zdenek
On 01/14/2015 09:34 AM, Zdenek Kabelac wrote:
Dne 14.1.2015 v 08:24 Bram Klein Gunnewiek napsal(a):
Hello,
I am trying to create snapshots of thick volumes that can't break because
they
overflow (thin volumes are not an option right now). I can't find
information
on how to calculate (in advance) the metadata overhead of a snapshot when
all
blocks in the origin volume are changed (e.g. the snapshot is 100% full). Is
there a way to do this?
Recent versions of lvm2 (let's say >= 111) are able to determine this size
automatically - just use: 'lvcreate -s -l100%ORIGIN vg/origin'
Regards
Zdenek
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