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ceph: make nearfull sync writes opt-in

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The kernel CephFS client has historically treated a cluster or pool
NEARFULL condition as a request to force successful writes through
generic_write_sync().  That effectively turns otherwise buffered writes
into synchronous writes and can cause a severe throughput drop as soon
as a single OSD or the file data pool crosses the nearfull threshold.

On modern large clusters, NEARFULL is primarily an operator health
signal rather than an immediate client-side capacity failure.  Operators
can still have substantial usable capacity while a cluster is
rebalancing, splitting PGs, or expanding onto new devices.  RBD, RGW and
the userspace CephFS client do not impose this extra client-side
sync-write throttle, so the kernel client behavior is surprising and
operationally painful.

Change the default behavior so NEARFULL no longer changes normal
write-sync semantics.  FULL and pool FULL still fail with -ENOSPC, and
explicitly synchronous writes continue to be synced by
generic_write_sync().

Add a nearfull_sync mount option for deployments that want the legacy
backpressure behavior.  When this option is set, successful writes are
promoted to IOCB_DSYNC if the cluster or file data pool is marked
NEARFULL, preserving the old behavior for conservative deployments.

Link: https://tracker.ceph.com/issues/74849
Signed-off-by: Alex Markuze <amarkuze@redhat.com>
Reviewed-by: Xiubo Li <xiubo.li@clyso.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
This commit is contained in:
Alex Markuze 2026-07-06 13:11:27 +00:00 • committed by Ilya Dryomov
commit 5f074d7f29
4 changed files with 22 additions and 3 deletions
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