Does this mean the space is allocated, but actually empty so can let’s say 
overwrite?

Istvan Szabo
Senior Infrastructure Engineer
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Agoda Services Co., Ltd.
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On 2021. Jan 28., at 19:51, Burkhard Linke 
<burkhard.li...@computational.bio.uni-giessen.de> wrote:

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Hi,

On 28.01.21 13:21, Szabo, Istvan (Agoda) wrote:
I mean the image hasn’t been deleted, but the content from the image.


RBD is (as the name implies) is a block device layer. Block devices do
not have a concept of content, file, directories or even allocated or
unallocated space. They are just continuous byte streams with a certain
size.


The _filesystem_ on top of the block device implements these concepts,
and also tracks free / unallocated space. And a filesystem is able to
inform the block layer about allocations using TRIM and DISCARD calls
(e.g. tools like fstrim or corresponding mount options). RBD usually
supports trim operation in recent kernels (krbd) or with certain
configuration options (librbd)


Regards,

Burkhard

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