mirror of
https://github.com/ceph/ceph
synced 2024-12-19 01:46:00 +00:00
91 lines
4.8 KiB
ReStructuredText
91 lines
4.8 KiB
ReStructuredText
==================================
|
|
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/assets/pdfs/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.
|
|
|
|
Export Process During Subtree Migration
|
|
---------------------------------------
|
|
|
|
Once the exporter verifies that the subtree is permissible to be exported
|
|
(Non degraded cluster, non-frozen subtree root), the subtree root
|
|
directory is temporarily auth pinned, the subtree freeze is initiated,
|
|
and the exporter is committed to the subtree migration, barring an
|
|
intervening failure of the importer or itself.
|
|
|
|
The MExportDiscover message is exchanged to ensure that the inode for the
|
|
base directory being exported is open on the destination node. It is
|
|
auth pinned by the importer to prevent it from being trimmed. This occurs
|
|
before the exporter completes the freeze of the subtree to ensure that
|
|
the importer is able to replicate the necessary metadata. When the
|
|
exporter receives the MDiscoverAck, it allows the freeze to proceed by
|
|
removing its temporary auth pin.
|
|
|
|
A warning stage occurs only if the base subtree directory is open by
|
|
nodes other than the importer and exporter. If it is not, then this
|
|
implies that no metadata within or nested beneath the subtree is
|
|
replicated by any node other than the importer and exporter. If it is,
|
|
then an MExportWarning message informs any bystanders that the
|
|
authority for the region is temporarily ambiguous, and lists both the
|
|
exporter and importer as authoritative MDS nodes. In particular,
|
|
bystanders who are trimming items from their cache must send
|
|
MCacheExpire messages to both the old and new authorities. This is
|
|
necessary to ensure that the surviving authority reliably receives all
|
|
expirations even if the importer or exporter fails. While the subtree
|
|
is frozen (on both the importer and exporter), expirations will not be
|
|
immediately processed; instead, they will be queued until the region
|
|
is unfrozen and it can be determined that the node is or is not
|
|
authoritative.
|
|
|
|
The exporter then packages an MExport message containing all metadata
|
|
of the subtree and flags the objects as non-authoritative. The MExport message sends
|
|
the actual subtree metadata to the importer. Upon receipt, the
|
|
importer inserts the data into its cache, marks all objects as
|
|
authoritative, and logs a copy of all metadata in an EImportStart
|
|
journal message. Once that has safely flushed, it replies with an
|
|
MExportAck. The exporter can now log an EExport journal entry, which
|
|
ultimately specifies that the export was a success. In the presence
|
|
of failures, it is the existence of the EExport entry only that
|
|
disambiguates authority during recovery.
|
|
|
|
Once logged, the exporter will send an MExportNotify to any
|
|
bystanders, informing them that the authority is no longer ambiguous
|
|
and cache expirations should be sent only to the new authority (the
|
|
importer). Once these are acknowledged back to the exporter,
|
|
implicitly flushing the bystander to exporter message streams of any
|
|
stray expiration notices, the exporter unfreezes the subtree, cleans
|
|
up its migration-related state, and sends a final MExportFinish to the
|
|
importer. Upon receipt, the importer logs an EImportFinish(true)
|
|
(noting locally that the export was indeed a success), unfreezes its
|
|
subtree, processes any queued cache expirations, and cleans up its
|
|
state.
|