Hi,
On 02/01/2018 10:43 AM, Konstantin Shalygin wrote:
Hi cephers.
I have typical double root crush - for nvme pools and hdd pools
created on Kraken cluster (what I mean:
http://cephnotes.ksperis.com/blog/2015/02/02/crushmap-example-of-a-hierarchical-cluster-map).
Now cluster upgraded to Luminous and going to devices classes crush
rules and I looking for experience.
1. Enable new crush rule with devices-class is safe for data and clients?
2. How much data movement? Should I be ready for slow requests?
We have changed our similar setup to a device class based one. According
to the documentation the device classes are implemented by 'shadow'
crush tree. 'ceph osd crush tree --show-shadow' displays all tree
including the device class specific ones. This allows the device class
setup to be backwards compatible with older releases.
We had a MASSIVE data movement upon changing the crush rules to device
class based one. I'm not sure about the exact reasons, but I assume that
the order of hosts in the crush tree has changed (hosts are ordered
lexically now...), which resulted in about 80% of data being moved around.
So be prepared for slow requests, and set the corresponding
configuration values to reduce the backfill impact.
Regards,
Burkhard
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