[PATCH manager 0/1] ceph: add opt-in locality-aware replica reads (crush_location_hook)

[Kefu Chai <k.chai@proxmox.com>]

This patch was prompted by a forum thread [1] in which a user reported
persistent high IO wait on PostgreSQL VMs running on a three-AZ Ceph
cluster. The discussion surfaced a general optimization opportunity:
librbd, by default, always reads from the primary OSD regardless of
its location. In a multi-AZ deployment, that can mean every read pays
a cross-AZ round-trip even when a same-AZ replica is available.

rbd_read_from_replica_policy = localize addresses this by directing
librbd to prefer the nearest replica, but it requires the client to
declare its own position in the CRUSH hierarchy. This patch ships a
hook script that supplies that position by querying the live CRUSH map
(ceph osd crush find), and wires it up as an opt-in in pveceph init.

The benefit scales with topology: in a multi-AZ cluster it keeps reads
within the same AZ; in a hyperconverged setup, reads to a co-located
OSD never leave the host at all. The feature is opt-in because it can
degrade performance when replicas are equidistant or when the hook
falls back to an incorrect CRUSH root — see the commit message for
details.

[1] https://forum.proxmox.com/threads/ceph-vm-with-high-io-wait.181751/
  

Kefu Chai (1):
  ceph: add opt-in locality-aware replica reads (crush_location_hook)

 PVE/API2/Ceph.pm                       | 17 ++++++++++
 bin/Makefile                           |  3 +-
 bin/ceph-crush-location                | 43 ++++++++++++++++++++++++++
 www/manager6/ceph/CephInstallWizard.js |  8 ++++-
 4 files changed, 69 insertions(+), 2 deletions(-)
 create mode 100644 bin/ceph-crush-location

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2.47.3