On 02/02/18 08:33, Kevin Olbrich wrote:
> Hi!
> I am planning a new Flash-based cluster. In the past we used SAMSUNG PM863a 
> 480G as journal drives in our HDD cluster.
> After a lot of tests with luminous and bluestore on HDD clusters, we plan to 
> re-deploy our whole RBD pool (OpenNebula cloud) using these disks.
> As far as I understand, it would be best to skip journaling / WAL and just 
> deploy every OSD 1-by-1. This would have the following pro's (correct me, if 
> I am wrong):
> - maximum performance as the journal is spread accross all devices
> - a lost drive does not affect any other drive
> Currently we are on CentOS 7 with elrepo 4.4.x-kernel. We plan to migrate to 
> Ubuntu 16.04.3 with HWE (kernel 4.10).
> Clients will be Fedora 27 + OpenNebula.
> Any comments?
> Thank you.
> Kind regards,
> Kevin

There is only a real advantage to separating the DB/WAL from the main data if 
they're going to be hosted on a device which is appreciably faster than the 
main storage. Since you're going all SSD, it makes sense to deploy each OSD 
all-in-one; as you say, you don't bottleneck on any one disk, and it also 
offers you more maintenance flexibility as you will be able to easily move OSDs 
between hosts if required. If you wanted to start pushing performance more, 
you'd be looking at putting NVMe disks in your hosts for DB/WAL.

FYI, the 16.04 HWE kernel has currently rolled on over to 4.13.


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