Hi,
I’d like to know how a file that’s been striped across multiple objects/object sets (potentially multiple placement groups) is reconstituted and returned back to a client?
For example say I have a 100 MB file, foo that’s been striped across 16 objects in 2 object sets. What is the data flow in terms of CRUSH to get back all the “bits” and return the reconstruct original file. Is there some meta involved?
By default files are simply chucked into 4MB pieces, so the client computes the location of each in turn (based on ino and offset in the FS, or RBD volume and offset, etc) and fetches it. More complicated striping strategies (which I believe you can lean about in the docs) might read part of a few objects several times before moving on to another set.
If you're interested in more you'll need more detailed questions. :)
-Greg
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Software Engineer #42 @ http://inktank.com | http://ceph.com
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