On Fri, 8 Oct 2010, Wido den Hollander wrote: > > Your example > > > > > step take root > > > step choose firstn 0 type rack > > > step choose firstn 1 type host > > > step choose firstn 2 type device > > > step emit > > > > would choose N(=3) racks, pick 1 host in each rack, and then pick 2 > > devices on each host (for a total of 6 devices). > > > > Does that make sense? > > Makes sense, so this should be fine: > > step take root > step choose firstn 0 type rack > step choose firstn 1 type host > step choose firstn 1 type device > step emit > > This way we find enough "racks" to store all my replicas. Yeah, this will work. But the 'chooseleaf' approach is still better, because the above rule might pick a rack that has no active devices, and end up with a smaller result. > Since my replication is set to three, that should be enough, since I > selected three hosts/devices. > > Is there any benefit from finding more devices then the replication > level? > > What would I benefit if I found 6 devices, where 3 should be enough? > Does CRUSH also select OSD's which are down? I'm not sure offhand what the system would do in that case. It might help mitigate the problems above, but I haven't checked. > Or should I just stick to "chooseleaf" in this situation? Yeah :) sage > > Wido > > > sage > > > > > > > > > > > > > > > > > That would choose N racks, and then for each rack, choose a nested device. > > > > The problem is when one of the racks it chooses has no (or few) online > > > > devices beneath it, we fail to find a usable device, and the result set > > > > will have <N devices. Chooseleaf doesn't have that problem. > > > > > > So chooseleaf rack should be safe enough in this case? > > > > > > > > > > > sage > > > > > > Wido > > > > > > -- > > > To unsubscribe from this list: send the line "unsubscribe ceph-devel" in > > > the body of a message to majordomo@xxxxxxxxxxxxxxx > > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > > > > > > > -- > > To unsubscribe from this list: send the line "unsubscribe ceph-devel" in > > the body of a message to majordomo@xxxxxxxxxxxxxxx > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > -- > To unsubscribe from this list: send the line "unsubscribe ceph-devel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > > -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
