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Chapter 19. Storing/Retrieving Object Data

To perform storage operations with Ceph Storage Cluster, all Ceph clients regardless of type must:

  1. Connect to the cluster.
  2. Create an I/O contest to a pool.
  3. Set an object name.
  4. Execute a read or write operation for the object.

The Ceph Client retrieves the latest cluster map and the CRUSH algorithm calculates how to map the object to a placement-group, and then calculates how to assign the placement group to a Ceph OSD Daemon dynamically. Client types such as Ceph Block Device and the Ceph Object Gateway perform the last two steps transparently.

To find the object location, all you need is the object name and the pool name. For example: 

ceph osd map <poolname> <object-name>
Note

The rados CLI tool in the following example is for Ceph administrators only.

Exercise: Locate an Object

As an exercise, lets create an object. Specify an object name, a path to a test file containing some object data and a pool name using the rados put command on the command line. For example: 

echo <Test-data> > testfile.txt
rados put <object-name> <file-path> --pool=<pool-name>
rados put test-object-1 testfile.txt --pool=data

To verify that the Ceph Storage Cluster stored the object, execute the following: 

rados -p data ls

Now, identify the object location: 

ceph osd map <pool-name> <object-name>
ceph osd map data test-object-1

Ceph should output the object’s location. For example: 

osdmap e537 pool 'data' (0) object 'test-object-1' -> pg 0.d1743484 (0.4) -> up [1,0] acting [1,0]

To remove the test object, simply delete it using the rados rm command. For example: 

rados rm test-object-1 --pool=data

As the cluster size changes, the object location may change dynamically. One benefit of Ceph’s dynamic rebalancing is that Ceph relieves you from having to perform the migration manually.