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[OpenStack Infra <2078476@xxxxxxxxxxxxxxxxxx>]

Reviewed:  https://review.opendev.org/c/openstack/glance/+/927844
Committed: https://opendev.org/openstack/glance/commit/39e407e9ffe956d40a261905ab98c13b5455e27d
Submitter: "Zuul (22348)"
Branch:    master

commit 39e407e9ffe956d40a261905ab98c13b5455e27d
Author: Cyril Roelandt <cyril@xxxxxxxxxx>
Date:   Tue Sep 3 17:25:54 2024 +0200

    Documentation: fix default value for rbd_store_chunk_size
    
    Closes-Bug: #2078476
    Change-Id: I3b83e57eebf306c4de28fd58589522970e62cf42


** Changed in: glance
       Status: In Progress => Fix Released

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https://bugs.launchpad.net/bugs/2078476

Title:
  rbd_store_chunk_size defaults to 8M not 4M

Status in Glance:
  Fix Released

Bug description:
  Versions affected: from current master to at least Antelope.

  The documentation
  (https://docs.openstack.org/glance/2024.1/configuration/configuring.html#configuring-
  the-rbd-storage-backend) states that the default rbd_store_chunk_size
  defaults to 4M while in reality it's 8M. This could have been 'only' a
  documentation bug, but there are two concerns here:

  1) Was it the original intention to have 8M chunk size (which is
  different from Ceph's defaults = 4M) or was it an inadvertent effect
  of other changes?

  2) Cinder defaults to rbd_store_chunk_size=4M. Having volumes created
  from Glance images results in an inherited chunk size of 8M (due to
  snapshotting) and could have unpredicted performance consequences. It
  feels like this scenario should at least be documented, if not
  avoided.

  Steps to reproduce:
  - deploy Glance with RBD backend enabled and default config;
  - query stores information for the configured chunk size (/v2/info/stores/detail)
  Optional:
  - have an image created in Ceph pool and validate its chunk size with rbd info command.

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