On 11/22/2011 12:11 AM, Atish Kathpal wrote:
Hello Does RADOS support metadata queries? I have gone through librados and the "rados" command line tool, and there doesn't seem to be a way for me to say, query the object store saying: "Give me all objects with Xattr Video = 1". The only way I can think of is to write some of my own code using librados.
That's right, RADOS operates on a single-object transaction level. There's no way for it to support consistent reads of multiple objects without rewriting much of the OSD - it's not designed for that. For your use case, you could store your metadata in a trivial map (tmap) object using librados' tmap_* functions. This is what radosgw uses for storing which objects are in which buckets. These are slow for large mappings, but we have plans to fix that (http://tracker.newdream.net/issues/1571).
Any pointers if the system has a metadata service or something that I can query? (I have seen few diagrams showing a metadata cluster as part of Ceph which supports metadata operations, how can I leverage that? reference diagram: http://www.usenix.org/event/osdi06/tech/full_papers/weil/weil_html/fig/overview.png)
The metadata cluster is for the Ceph distruted file system, which is another layer on top of RADOS.
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