doc/ceph-volume: add spillover fix procedure

Add a procedure that explains how, after an upgrade, to move bytes that
have spilled over to a relatively slow device back to the faster device.

This procedure was developed by Chris Dunlop on the [ceph-users] mailing
list, here: https://lists.ceph.io/hyperkitty/list/[email protected]/message/POPUFSZGXR3P2RPYPJ4WJ4HGHZ3QESF6/

Eugen Block requested the addition of this procedure to the
documentation on 30 Aug 2024.

Co-authored-by: Anthony D'Atri <[email protected]>
Signed-off-by: Zac Dover <[email protected]>

Author: zdover23

doc/ceph-volume/lvm/newdb.rst

@@ -9,3 +9,48 @@ Logical volume name format is vg/lv. Fails if OSD has already got attached DB.
 Attach vgname/lvname as a DB volume to OSD 1::
 
     ceph-volume lvm new-db --osd-id 1 --osd-fsid 55BD4219-16A7-4037-BC20-0F158EFCC83D --target vgname/new_db
+
+Reversing BlueFS Spillover to Slow Devices
+------------------------------------------
+
+Under certain circumstances, OSD RocksDB databases spill onto slow storage and
+the Ceph cluster returns specifics regarding BlueFS spillover warnings. ``ceph
+health detail`` returns these spillover warnings.  Here is an example of a
+spillover warning::
+
+   osd.76 spilled over 128 KiB metadata from 'db' device (56 GiB used of 60 GiB) to slow device
+
+To move this DB metadata from the slower device to the faster device, take the
+following steps:
+
+#. Expand the database's logical volume (LV):
+
+   .. prompt:: bash #
+
+      lvextend -l ${size} ${lv}/${db} ${ssd_dev}
+
+#. Stop the OSD:
+
+   .. prompt:: bash #
+
+      cephadm unit --fsid $cid --name osd.${osd} stop
+
+#. Run the ``bluefs-bdev-expand`` command:
+
+   .. prompt:: bash #
+
+      cephadm shell --fsid $cid --name osd.${osd} -- ceph-bluestore-tool bluefs-bdev-expand --path /var/lib/ceph/osd/ceph-${osd}
+
+#. Run the ``bluefs-bdev-migrate`` command:
+
+   .. prompt:: bash #
+
+      cephadm shell --fsid $cid --name osd.${osd} -- ceph-bluestore-tool bluefs-bdev-migrate --path /var/lib/ceph/osd/ceph-${osd} --devs-source /var/lib/ceph/osd/ceph-${osd}/block --dev-target /var/lib/ceph/osd/ceph-${osd}/block.db 
+
+#. Restart the OSD:
+
+   .. prompt:: bash #
+
+      cephadm unit --fsid $cid --name osd.${osd} start
+
+.. note:: *The above procedure was developed by Chris Dunlop on the [ceph-users] mailing list, and can be seen in its original context here:* `[ceph-users] Re: Fixing BlueFS spillover (pacific 16.2.14) <https://lists.ceph.io/hyperkitty/list/[email protected]/message/POPUFSZGXR3P2RPYPJ4WJ4HGHZ3QESF6/>`_