doc/man: improve bluefs-bdev-expand option

Signed-off-by: Kefu Chai <kchai@redhat.com>

diff --git a/doc/man/8/ceph-bluestore-tool.rst b/doc/man/8/ceph-bluestore-tool.rst
index 0ad5c20c3d6089594395ec377358ecdca5e2c54d..2bc951ebd13c95459d61985c226f36d7311bb1fa 100644 (file)
--- a/doc/man/8/ceph-bluestore-tool.rst
+++ b/doc/man/8/ceph-bluestore-tool.rst
@@ -48,7 +48,7 @@ Commands
 
 :command:`bluefs-export`
 
-   Export the contents of BlueFS (i.e., rocksdb files) to an output directory.
+   Export the contents of BlueFS (i.e., RocksDB files) to an output directory.
 
 :command:`bluefs-bdev-sizes` --path *osd path*
 
@@ -56,7 +56,13 @@ Commands
 
 :command:`bluefs-bdev-expand` --path *osd path*
 
-   Instruct BlueFS to check the size of its block devices and, if they have expanded, make use of the additional space.
+   Instruct BlueFS to check the size of its block devices and, if they have
+   expanded, make use of the additional space. Please note that only the new
+   files created by BlueFS will be allocated on the preferred block device if
+   it has enough free space, and the existing files that have spilled over to
+   the slow device will be gradually removed when RocksDB performs compaction.
+   In other words, if there is any data spilled over to the slow device, it
+   will be moved to the fast device over time.
 
 :command:`bluefs-bdev-new-wal` --path *osd path* --dev-target *new-device*