--- /dev/null
+==================================
+CephFS Dynamic Metadata Management
+==================================
+Metadata operations usually take up more than 50 percent of all
+file system operations. Also the metadata scales in a more complex
+fashion when compared to scaling storage (which in turn scales I/O
+throughput linearly). This is due to the hierarchical and
+interdependent nature of the file system metadata. So in CephFS,
+the metadata workload is decoupled from data workload so as to
+avoid placing unnecessary strain on the RADOS cluster. The metadata
+is hence handled by a cluster of Metadata Servers (MDSs).
+CephFS distributes metadata across MDSs via `Dynamic Subtree Partitioning <https://ceph.com/wp-content/uploads/2016/08/weil-mds-sc04.pdf>`__.
+
+Dynamic Subtree Partitioning
+----------------------------
+In traditional subtree partitioning, subtrees of the file system
+hierarchy are assigned to individual MDSs. This metadata distribution
+strategy provides good hierarchical locality, linear growth of
+cache and horizontal scaling across MDSs and a fairly good distribution
+of metadata across MDSs.
+
+.. image:: subtree-partitioning.svg
+
+The problem with traditional subtree partitioning is that the workload
+growth by depth (across a single MDS) leads to a hotspot of activity.
+This results in lack of vertical scaling and wastage of non-busy resources/MDSs.
+
+This led to the adoption of a more dynamic way of handling
+metadata: Dynamic Subtree Partitioning, where load intensive portions
+of the directory hierarchy from busy MDSs are migrated to non busy MDSs.
+
+This strategy ensures that activity hotspots are relieved as they
+appear and so leads to vertical scaling of the metadata workload in
+addition to horizontal scaling.
projects / ceph-ci.git / commitdiff
