The most important thing to remember about CRUSH is that the H stands for hashing.
If you hash the same object you're going to get the same result. e.g. cat /etc/fstab | md5sum is always the same output, unless you change the file contents. CRUSH uses the number of osds and the object and the pool and a bunch of other things to create a hash which determines placement. If any of that changes then the hash will change, and the placement with change, if it restores to exactly how it was, then the placement returns to how it was. On Thu, 2018-12-06 at 09:44 +0100, Marc Roos wrote: Afaik it is not random, it is calculated where your objects are stored. Some algorithm that probably takes into account how many osd's you have and their sizes. How can it be random placed? You would not be able to ever find it again. Because there is not such a thing as a 'file allocation table' But better search for this, I am not that deep into ceph ;) -----Original Message----- From: Franck Desjeunes [mailto: <mailto:fdesjeu...@gmail.com> fdesjeu...@gmail.com ] Sent: 06 December 2018 08:01 To: <mailto:ceph-users@lists.ceph.com> ceph-users@lists.ceph.com Subject: [ceph-users] Crush, data placement and randomness Hi all cephers. I don't know if this is the right place to ask this kind of questions, but I'll give it a try. I'm getting interested in ceph and deep dived into the technical details of it but I'm struggling to understand few things. When I execute a ceph osd map on an hypothetic object that does not exist, the command always give me the same OSDs set to store the object. So, what is the randomness of the CRUSH algorithm if an object A will always be stored in the same OSDs set ? In the same way, why when I use librados to read an object, the stack trace shows that the code goes through the exact same functions calls as to create an object to get the OSDs set ? As far as I see, for me, CRUSH is fully deterministic and I don't understand why it is qualified as a pseudo-random algorithm. Thank you for your help. Best regards. _______________________________________________ ceph-users mailing list <mailto:ceph-users@lists.ceph.com> ceph-users@lists.ceph.com <http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com -- Leon L. Robinson <leon.robin...@ukfast.co.uk<mailto:leon.robin...@ukfast.co.uk>> ________________________________ NOTICE AND DISCLAIMER This e-mail (including any attachments) is intended for the above-named person(s). If you are not the intended recipient, notify the sender immediately, delete this email from your system and do not disclose or use for any purpose. We may monitor all incoming and outgoing emails in line with current legislation. We have taken steps to ensure that this email and attachments are free from any virus, but it remains your responsibility to ensure that viruses do not adversely affect you
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