Merge pull request #51978 from sseshasa/wip-61624-quincy

quincy: PendingReleaseNotes: Document mClock scheduler fixes and enhancements

Reviewed-by: Neha Ojha <nojha@redhat.com>

diff --cc PendingReleaseNotes
index 6d576173b1eaf586f7be3b31ab2c184cf6b6ce58,58b37672d68613c07f15a9563723c1546e3a63d3..cbdc83fc8a28efc690ff0b9b5a1c910f9da5d852
--- 1/PendingReleaseNotes
--- 2/PendingReleaseNotes
+++ b/PendingReleaseNotes
@@@ -5,10 -5,30 +5,33 @@@
    registered a RADOS client in the `name` field added to elements of the
    `active_clients` array. Previously, only the address of a module's RADOS
    client was shown in the `active_clients` array.
+ * mClock Scheduler: The mClock scheduler (default scheduler in Quincy) has
+   undergone significant usability and design improvements to address the slow
+   backfill issue. Some important changes are:
+   * The 'balanced' profile is set as the default mClock profile because it
+     represents a compromise between prioritizing client IO or recovery IO. Users
+     can then choose either the 'high_client_ops' profile to prioritize client IO
+     or the 'high_recovery_ops' profile to prioritize recovery IO.
+   * QoS parameters like reservation and limit are now specified in terms of a
+     fraction (range: 0.0 to 1.0) of the OSD's IOPS capacity.
+   * The cost parameters (osd_mclock_cost_per_io_usec_* and
+     osd_mclock_cost_per_byte_usec_*) have been removed. The cost of an operation
+     is now determined using the random IOPS and maximum sequential bandwidth
+     capability of the OSD's underlying device.
+   * Degraded object recovery is given higher priority when compared to misplaced
+     object recovery because degraded objects present a data safety issue not
+     present with objects that are merely misplaced. Therefore, backfilling
+     operations with the 'balanced' and 'high_client_ops' mClock profiles may
+     progress slower than what was seen with the 'WeightedPriorityQueue' (WPQ)
+     scheduler.
+   * The QoS allocations in all the mClock profiles are optimized based on the above
+     fixes and enhancements.
+   * For more detailed information see:
+     https://docs.ceph.com/en/quincy/rados/configuration/mclock-config-ref/
  
 +* CEPHFS: After recovering a Ceph File System post following the disaster recovery
 +  procedure, the recovered files under `lost+found` directory can now be deleted.
 +
  >=17.2.6
  --------