Key/Value Store in LINSTOR

Recently we introduced a Key/Value store in LINSTOR and exposed it in a developer-friendly way in the Python API (python-linstor). The first question is why would one want such a Key/Value store in LINSTOR when there are many high performance implementations such as etcd. The request for a K/V store was mainly driven by LINSTOR plugin developers. For example many plugins need to store some kind of meta data like a description for a resource. Existing, non-LINSTOR plugins sometimes store such information in a local json file or in a file per resource. This, on one hand, is clumsy and on the other hand in a distributed system like DRBD/LINSTOR, the data needs be available on all nodes.

In LINSTOR a K/V store has a unique name (e.g., one per plugin) and it can store up to 510 bytes for a key, and 4096 bytes for the value. The implementation in python-linstor provides an interface that mimics a Python3 dictionary. In addition to the discussed unique name, the K/V store as implemented in the Python library also provides so called namespaces. One can think of a namespace as a UNIX directory structure where components of a path (i.e., the namespace) are separated by a /. In the following we show an example using the Python library:

import linstor
kv = linstor.KV('myKV', namespace='/foo/bar/')
kv['key'] = 'val'
list(kv.items()) -> [('key', 'val')]
kv.namespace = '/'
list(kv.items()) -> [('/foo/bar/key', 'val')]
kv['foo/baz/key'] = 'valbaz'
kv.namespace = '/foo/bar'
list(kv.items()) -> [('key', 'val')] # keys in /foo/baz not visible

Key/Value Store makes life easier

Developers already familiar with LINSTOR details might know there is a concept that sounds similar to what the K/V store can do, the so called “AUX props”. One can attach meta data to basically every LINSTOR object. While they sound similar, there are noteworthy differences:

• An AUX prop is tied to the according object. When the object is gone, the meta data is gone. This might be desired and can be an advantage.

• The K/V store exists as long as the LINSTOR cluster exists. Data is not attached to another LINSTOR object. Depending on the situation this might be an advantage compared to a plain AUX property.

• The K/V store has a much nicer interface. It just behaves like a Python dictionary.

• The K/V store and its namespace implementation make it a lot easier to store hierarchical data.

• Searching AUX props can be difficult: For example to find a specific AUX prop set on volume definition, one would have to iterate over the AUX props of.

All in all the K/V store makes the life of a plugin developer much easier. BTW: The text of this blog post easily fits into a single K/V pair 😀

Software Engineer at Linbit
Roland Kammerer studied technical computer science at the Vienna University of Technology and graduated with distinction. Currently, he is a PhD candidate with a research focus on time-triggered realtime-systems and works for LINBIT in the DRBD development team.

Like? Share it with the world.

Share on facebook
Share on twitter
Share on linkedin
Share on whatsapp
Share on vk
Share on reddit
Share on email