On Mon, Mar 1, 2021 at 1:35 PM Pawel S <pejotes@xxxxxxxxx> wrote: > > hello! > > I'm trying to understand how Bluestore cooperates with RBD image clones, so > my test is simple > > 1. create an image (2G) and fill with data > 2. create a snapshot > 3. protect it > 4. create a clone of the image > 5. write a small portion of data (4K) to clone > 6. check how it changed and if just 4K are used to prove CoW allocated new > extent instead of copying out snapped data. > > Unfortunately it occurs that at least rbd du reports that 4M was changed > and the clone consumes 4M of data instead of expected 4K... > ''' > rbd du rbd/clone1 > NAME PROVISIONED USED > clone1 2 GiB 4 MiB > ''' > > How can I trace/prove Bluestore CoW really works in this case, and prevent > copying the rest of the 4M stripe like Filestore did ? RBD clones are not the same as RBD snapshots. Writing to an unallocated extent within an RBD clone image will always copy up to the "object-size" amount of data from the parent image. In that respect, there is no difference between Bluestore and Filestore since this logic is agnostic to the underlying OSD object store. RBD snapshots, however, do result in copy-on-write like semantics within Bluestore, although it's technically a redirect-on-write since older data is not moved to preserve the snapshot history and instead the new write request is redirected to a newly allocated block within Bluestore and metadata is updated to reference the new block. > p.s tested on Luminous/Octopus, ssd devices, min_alloc_size: 16k, > block_size: 4k > > best regards! > -- > Pawel S. > _______________________________________________ > ceph-users mailing list -- ceph-users@xxxxxxx > To unsubscribe send an email to ceph-users-leave@xxxxxxx > -- Jason _______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx To unsubscribe send an email to ceph-users-leave@xxxxxxx
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