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PendingReleaseNotes: Document mClock scheduler fixes and enhancements

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Signed-off-by: Sridhar Seshasayee <sseshasa@redhat.com>
(cherry picked from commit 49aad118aa)

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- Modified doc URL to point to reef
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Sridhar Seshasayee 2023-06-05 13:41:28 +05:30
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@ -139,6 +139,29 @@
* CEPHFS: After recovering a Ceph File System post following the disaster recovery
procedure, the recovered files under `lost+found` directory can now be deleted.
* core: cache-tiering is now deprecated.
* 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/reef/rados/configuration/mclock-config-ref/
>=17.2.1