Because the snapshot is the full status of the rbd at the time it was created, which was 100MB. Any modifications made to the rbd after the snapshot will show up in the rbds size until about snapshot is taken. If you delete the snapshot, it rolls up where the size was reported.
Basically the oldest point in time an object was modified in an rbd/snapshots will own the size of the object. The end itself is the current state, so any object not modified since the last snapshot will not count towards the size of the current rbd.
On Wed, Jul 26, 2017, 3:00 PM Laszlo Budai <laszlo@xxxxxxxxxxxxxxxx> wrote:
Dear all,
Where can I read more about how the space used by a snapshot of an RBD image is calculated? Or can someone explain it here?
I can see that before the snapshot is created, the size of the image is let's say 100M as reported by the rbd du command, while after taking the snapshot, I can see in the rbd du output that the 100M appears to belong to the snapshot, and the image is only consuming a small amount. How comes?
Thank you,
Laszlo
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