Hi again.
It seems I've found the problem, although I don't understand the root
cause.
I looked into OSD datastore using ceph-objectstore-tool and I see that
for almost every object there are two copies, like:
2#13:080008d8:::rbd_data.15.3d3e1d6b8b4567.0000000000361a96:28#
2#13:080008d8:::rbd_data.15.3d3e1d6b8b4567.0000000000361a96:head#
And more interesting is the fact that these two copies don't differ (!).
So the space is taken up by the unneeded snapshot copies.
rbd_data.15.3d3e1d6b8b4567 is the prefix of the biggest (14 TB) base
image we have. This image has 1 snapshot:
[root@sill-01 ~]# rbd info rpool_hdd/rms-201807-golden
rbd image 'rms-201807-golden':
size 14 TiB in 3670016 objects
order 22 (4 MiB objects)
id: 3d3e1d6b8b4567
data_pool: ecpool_hdd
block_name_prefix: rbd_data.15.3d3e1d6b8b4567
format: 2
features: layering, exclusive-lock, object-map, fast-diff,
deep-flatten, data-pool
op_features:
flags:
create_timestamp: Tue Aug 7 13:00:10 2018
[root@sill-01 ~]# rbd snap ls rpool_hdd/rms-201807-golden
SNAPID NAME SIZE TIMESTAMP
37 initial 14 TiB Tue Aug 14 12:42:48 2018
The problem is this image has NEVER been written to after importing it
to Ceph with RBD. All writes go only to its clones.
So I have 2.. no, 5 questions:
1) Why base image snapshot is "provisioned" while the image isn't
written to? May it be related to `rbd snap revert`? (i.e. does rbd snap
revert just copy all snapshot data into the image itself?)
2) If all parent snapshots seem to be forcefully provisioned on write:
Is there a way to disable this behaviour? Maybe if I make the base image
readonly its snapshots will stop to be "provisioned"?
3) Even if there is no way to disable it: why does Ceph create extra
copy of equal snapshot data during rebalance?
4) What's ":28" in rados objects? Snapshot id is 37. Even in hex 0x28 =
40, not 37. Or does RADOS snapshot id not need to be equal to RBD
snapshot ID?
5) Am I safe to "unprovision" the snapshot? (for example, by doing `rbd
snap revert`?)
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