In an earlier post we shed some light on Arakoon, our own always consistent distributed key-valuedatabase. Arakoon is used in many parts of the Open vStorage platform. One of the use cases is to store the metadata of the native ALBA object store. Do note that ALBA is NOT a general purpose object store but specifically crafted and optimized for Open vStorage. ALBA uses a collection of Arakoon databases to store where and how objects are stored on the disks in the backend. Typically the SCOs and TLogs of each vDisk end up in a separate bucket, a namespace, on the backend. For each object in the namespace there is a manifest that describes where and how the object is stored on the backend. To glue the namespaces, the manifests and the disks in the backend together, ALBA uses 2 types of Arakoon databases: the ALBA Backend Manager (ABM) and one or more NameSpace Managers (NSM).
The ALBA Manager (ABM) is the entry point for all ALBA clients which want to store or retrieve something from the backend. The ALBA Manager DB knows which physical disks belong to the backend, which namespaces exist and on which NSM hosts they can be found.
To optimize the Arakoon DB it is loaded with the albamgr plugin, a collection of specific ABM user functions. Typically there is only a single ABM manager in a cluster.
A NameSpace Manager (NSM) is an Arakoon cluster which holds the manifests for the namespaces assigned to the NSM. Which NSM is managing which namespaces is registered with the ALBA Manager. The NSM is also the remote API offered by the NSM host to manipulate most of the object metadata during normal operation. Its coordinates can be retrieved from the ALBA Manager by (proxy) clients and maintenance agents.
To optimize the Arakoon DB it is loaded with the nsm_host plugin, a collection of specific NSM host user functions. Typically there are multiple NSM clusters for a single ALBA backend. This allows to scale the backend both capacity and performance wise.
Let’s have a look at the IO path. Whenever the Volume Driver needs to store an object on the backend, a SCO or a TLog, it hands the object to one of the ALBA proxies on the same host. The ALBA proxy contains an ALBA client which communicates with the ABM to know on which NSM and disks it can store the object. Once the object is stored on the disks, the manifest with the metadata is registered in the NSM. For performance reasons the different fragment of the object and the manifest can be cached by the ALBA proxy.
In case the Volume Driver needs data from the backend, because it is no longer in the write buffer, it request the proxy to fetch the exact data by asked for a SCO location and offset. In case the right fragment are in the fragment cache, the proxy returns the data immediately to the Volume Driver. Otherwise it can use the manifest from the cache or the manifest isn’t in the cache, the proxy contacts the ABM to get the right NSM and from that the manifest. Based upon the manifest the ALBA client fetches the data it needs from the physical disks and provides it to the Volume Driver.