https://ceph.com/wp-content/uploads/2016/08/weil-crush-sc06.pdf On Thu, Dec 6, 2018 at 8:11 PM Leon Robinson <leon.robin...@ukfast.co.uk> wrote: > > 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: > > fdesjeu...@gmail.com > > ] > > Sent: 06 December 2018 08:01 > > To: > > 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 > > ceph-users@lists.ceph.com > > > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com > > > -- > > Leon L. Robinson <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 > _______________________________________________ > ceph-users mailing list > ceph-users@lists.ceph.com > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
-- Cheers, Brad _______________________________________________ ceph-users mailing list ceph-users@lists.ceph.com http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
