1 Mar
2021
1 Mar
'21
8:16 p.m.
On Mon, Mar 1, 2021 at 3:07 PM Pawel S <pejotes(a)gmail.com> wrote:
--
Jason
Hello Jason!
On Mon, Mar 1, 2021, 19:48 Jason Dillaman <jdillama(a)redhat.com> wrote:
Yes, although since the Octopus release this copy-up from parent to
child operation is now a sparse-read from parent/sparse-write to child
(i.e. if the overlapping parent object extent was empty, no data would
need to be copied to the child). However, in your test case where
there is a full 4MiB of data in the overlapping parent extent, it
would all need to be copied to the child.
On Mon, Mar 1, 2021 at 1:35 PM Pawel S
<pejotes(a)gmail.com> wrote:
Thanks for clarification, so the only way to reduce impact here is to
decrease size of objects ?
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.
Thanks!!! So I've wrongly adopted the same for clones.
--
Pawel S.
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