The rules in the CRUSH map will always be followed. It is not possible for Ceph to go against that and put data into a root that shouldn't have it.
The problem with a cache tier is that Ceph is going to need to promote and evict stuff all the time (not free). A lot of people that want to use SSD cache tiering for RBDs end up with slower performance because of this. Christian Balzer is the expert on Cache Tiers for RBD usage. His primary stance is that it's most likely a bad idea, but there are definite cases where it's perfect.
On Thu, Aug 17, 2017 at 4:20 PM Oscar Segarra <oscar.segarra@xxxxxxxxx> wrote:
Hi David,Thanks a lot for your quick answer!If I'm understanding you correctly, you want to have 2 different roots that pools can be made using. The first being entirely SSD storage. The second being HDD Storage with an SSD cache tier on top of it.--> Yes, this is what I mean.--> I'm not an expert in CRUSH rules... Whit this configuration, it is guaranteed that objects stored in ssd pool do not "go" to the hdd disks?The above guide explains how to set up the HDD root and the SSD root. After that all you do is create a pool on the HDD root for RBDs, a pool on the SSD root for a cache tier to use with the HDD pool, and then a a pool on the SSD root for RBDs. There aren't actually a lot of use cases out there where using an SSD cache tier on top of an HDD RBD pool is what you really want. I would recommend testing this thoroughly and comparing your performance to just a standard HDD pool for RBDs without a cache tier.--> I'm working on a VDI solution where there are BASE IMAGES (raw) and qcow2 linked clones... where I expect not all VDIs to be powered on at the same time and perform a configuration to avoid problems related to login storm. (1000 hosts)--> Do you think it is not a good idea? do you know what does usually people configure for this kind of scenarios?Thanks a lot.2017-08-17 21:31 GMT+02:00 David Turner <drakonstein@xxxxxxxxx>:If I'm understanding you correctly, you want to have 2 different roots that pools can be made using. The first being entirely SSD storage. The second being HDD Storage with an SSD cache tier on top of it.The above guide explains how to set up the HDD root and the SSD root. After that all you do is create a pool on the HDD root for RBDs, a pool on the SSD root for a cache tier to use with the HDD pool, and then a a pool on the SSD root for RBDs. There aren't actually a lot of use cases out there where using an SSD cache tier on top of an HDD RBD pool is what you really want. I would recommend testing this thoroughly and comparing your performance to just a standard HDD pool for RBDs without a cache tier.On Thu, Aug 17, 2017 at 3:18 PM Oscar Segarra <oscar.segarra@xxxxxxxxx> wrote:_______________________________________________Hi,Sorry guys, during theese days I'm asking a lot about how to distribute my data.I have two kinds of VM:1.- Management VMs (linux) --> Full SSD dedicated disks2.- Windows VM --> SSD + HHD (with tiering).I'm working on installing two clusters on the same host but I'm encountering lots of problems as named clusters look not be fully supported.In the same cluster, Is there any way to distribute my VMs as I like?Thanks a lot!Ó.
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