Chapter 3. Client Architecture

Modern applications need a simple object storage interface with asynchronous communication capability. The Ceph Storage Cluster provides a simple object storage interface with asynchronous communication capability. The interface provides direct, parallel access to objects throughout the cluster.

A Ceph client can register a persistent interest with an object and keep a session to the primary OSD open. The client can send a notification message and payload to all watchers and receive notification when the watchers receive the notification. This enables a client to use any object as a synchronization/communication channel.

Mandatory Exclusive Locks is a feature that locks an RBD to a single client, if multiple mounts are in place. This helps address the write conflict situation when multiple mounted client try to write to the same object. This feature is built on object-watch-notify explained in the previous section. So, when writing, if one client first establishes an exclusive lock on an object, another mounted client will first check to see if a peer has placed a lock on the object before writing.

With this feature enabled, only one client can modify an RBD device at a time, especially when changing internal RBD structures during operations like snapshot create/delete. It also provides some protection for failed clients. For instance, if a virtual machine seems to be unresponsive and you start a copy of it with the same disk elsewhere, the first one will be blacklisted in Ceph and unable to corrupt the new one.

Mandatory Exclusive Locks is not enabled by default. You have to explicitly enable it with --image-features parameter when creating an image. For example:

rbd -p mypool create myimage --size 102400 --image-features 5

Here, the numeral 5 is a summation of 1 and 4 where 1 enables layering support and 4 enables exclusive locking support. So, the above command will create a 100 GB rbd image, enable layering and exclusive lock.

Mandatory Exclusive Locks is also a prerequisite for object map. Without enabling exclusive locking support, object map support cannot be enabled.

Mandatory Exclusive Locks also does some ground work for mirroring.

Object map is a feature that tracks the presence of backing RADOS objects when a client writes to an rbd image. When a write occurs, that write is translated to an offset within a backing RADOS object. When the object map feature is enabled, the presence of these RADOS objects is tracked. So, we can know if the objects actually exist. Object map is kept in-memory on the librbd client so it can avoid querying the OSDs for objects that it knows don’t exist. In other words, object map is an index of the objects that actually exists.

Object map is beneficial for certain operations, viz:

A shrink resize operation is like a partial delete where the trailing objects are deleted.

An export operation knows which objects are to be requested from RADOS.

A copy operation knows which objects exist and need to be copied. It does not have to iterate over potentially hundreds and thousands of possible objects.

A flatten operation performs a copy-up for all parent objects to the clone so that the clone can be detached from the parent i.e, the reference from the child clone to the parent snapshot can be removed. So, instead of all potential objects, copy-up is done only for the objects that exist.

A delete operation deletes only the objects that exist in the image.

A read operation skips the read for objects it knows doesn’t exist.

So, for operations like resize (shrinking only), exporting, copying, flattening, and deleting, these operations would need to issue an operation for all potentially affected RADOS objects (whether they exist or not). With object map enabled, if the object doesn’t exist, the operation need not be issued.

For example, if we have a 1 TB sparse RBD image, it can have hundreds and thousands of backing RADOS objects. A delete operation without object map enabled would need to issue a remove object operation for each potential object in the image. But if object map is enabled, it only needs to issue remove object operations for the objects that exist.

Object map is valuable against clones that don’t have actual objects but gets object from parent. When there is a cloned image, the clone initially has no objects and all reads are redirected to the parent. So, object map can improve reads as without the object map, first it needs to issue a read operation to the OSD for the clone, when that fails, it issues another read to the parent — with object map enabled. It skips the read for objects it knows doesn’t exist.

Object map is not enabled by default. You have to explicitly enable it with --image-features parameter when creating an image. Also, Mandatory Exclusive Locks (mentioned in previous section) is a prerequisite for object map. Without enabling exclusive locking support, object map support cannot be enabled. To enable object map support when creating a image, execute:

rbd -p mypool create myimage --size 102400 --image-features 13

Here, the numeral 13 is a summation of 1, 4 and 8 where 1 enables layering support, 4 enables exclusive locking support and 8 enables object map support. So, the above command will create a 100 GB rbd image, enable layering, exclusive lock and object map.

Storage devices have throughput limitations, which impact performance and scalability. So storage systems often support striping—​storing sequential pieces of information across across multiple storage devices—​to increase throughput and performance. The most common form of data striping comes from RAID. The RAID type most similar to Ceph’s striping is RAID 0, or a 'striped volume.' Ceph’s striping offers the throughput of RAID 0 striping, the reliability of n-way RAID mirroring and faster recovery.

Ceph provides three types of clients: Ceph Block Device, Ceph Filesystem, and Ceph Object Storage. A Ceph Client converts its data from the representation format it provides to its users (a block device image, RESTful objects, CephFS filesystem directories) into objects for storage in the Ceph Storage Cluster.

The simplest Ceph striping format involves a stripe count of 1 object. Ceph Clients write stripe units to a Ceph Storage Cluster object until the object is at its maximum capacity, and then create another object for additional stripes of data. The simplest form of striping may be sufficient for small block device images, S3 or Swift objects. However, this simple form doesn’t take maximum advantage of Ceph’s ability to distribute data across placement groups, and consequently doesn’t improve performance very much. The following diagram depicts the simplest form of striping:

If you anticipate large images sizes, large S3 or Swift objects (e.g., video), you may see considerable read/write performance improvements by striping client data over multiple objects within an object set. Significant write performance occurs when the client writes the stripe units to their corresponding objects in parallel. Since objects get mapped to different placement groups and further mapped to different OSDs, each write occurs in parallel at the maximum write speed. A write to a single disk would be limited by the head movement (e.g. 6ms per seek) and bandwidth of that one device (e.g. 100MB/s). By spreading that write over multiple objects (which map to different placement groups and OSDs) Ceph can reduce the number of seeks per drive and combine the throughput of multiple drives to achieve much faster write (or read) speeds.

In the following diagram, client data gets striped across an object set (object set 1 in the following diagram) consisting of 4 objects, where the first stripe unit is stripe unit 0 in object 0, and the fourth stripe unit is stripe unit 3 in object 3. After writing the fourth stripe, the client determines if the object set is full. If the object set is not full, the client begins writing a stripe to the first object again (object 0 in the following diagram). If the object set is full, the client creates a new object set (object set 2 in the following diagram), and begins writing to the first stripe (stripe unit 16) in the first object in the new object set (object 4 in the diagram below).

Three important variables determine how Ceph stripes data:

Once the Ceph Client has striped data to stripe units and mapped the stripe units to objects, Ceph’s CRUSH algorithm maps the objects to placement groups, and the placement groups to Ceph OSD Daemons before the objects are stored as files on a storage disk.