Ceph client object map

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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 converted to an offset within a backing RADOS object. When the object map feature is enabled, the presence of these RADOS objects is tracked, allowing Ceph to 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 exist.

Object map is beneficial for certain operations, viz:

  • Resize
  • Export
  • Copy
  • Flatten
  • Delete
  • Read

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 get objects from parents. 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 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, run:

Example 

[root@mon ~]# rbd create --size 102400 mypool/myimage --image-feature 13

In this example, 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 command creates a 100 GB RBD image, enable layering, exclusive lock and object map.