The second id is to overlay device types in the crush map. In the olden days, we would define two seperate roots, one for hdd and another for SSD (each host would have a duplicate entry with a slightly different name and different id), then have a crush rule use the different root for the type of storage to use.
Now, you can just use device types for the same thing and tell the crush rule to only pick certain device types. If memory serves me, you can add device type to the osd line to put it in the device group, or just do it in the command line.
It also allows you to move from the old multi-root style to the new device style without data movement by making sure the IDs are consolidated properly.
On Thu, Dec 5, 2019, 11:46 AM Rodrigo Severo - Fábrica <rodrigo@xxxxxxxxxxxxxxxxxxx> wrote:
Hi,
The crushmap produced by ceph osd getcrushmap in ceph version 14.2.4
has more info than defined in
https://docs.ceph.com/docs/cuttlefish/rados/operations/crush-map/
There is a second id per bucket:
host a1-df {
id -3 # do not change unnecessarily
id -4 class hdd # do not change unnecessarily
# weight 1.819
alg straw2
hash 0 # rjenkins1
item osd.0 weight 1.819
}
What exactly is this second id which, by the way, comes with a device
class specified?
Regards,
Rodrigo Severo
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