Martin,
It all depends on your workload.
For example, if you are not bothered about write speed at all, I would say to configure primary affinity of your cluster properly so that primary OSDs can be the one hosted by SSDs..If you are considering 4 SSDs per node, so, total of 56 SSDs and 14 * 12 = 168 HDDs , I guess numbers should work out reasonably (considering 1 OSD per disk) well. This should give your cluster all SSD like read performance, but, write performance won’t improve (will be HDD like).
In this case, making 2 or 3 all SSD nodes with high performance servers make sense as all the read traffic will be landing there and with SSDs you need more powerful cpu complex.
If your workload is read/write mix, I would say your theory of 2 SSDs for journal and 2 for Cache pool make sense. Journal will help only for write and cache tier can help for read. But, I must say I am yet to evaluate cache tiering performance though.
In this case as you said, distributing the ssds across all nodes should be your correct approach.
Hope this helps,
Thanks & Regards
Somnath
From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx] On Behalf Of Martin Palma
Sent: Saturday, May 30, 2015 1:37 AM
To: ceph-users@xxxxxxxxxxxxxx
Subject: SSD disk distribution
Hello,
We are planing to deploy our first Ceph cluster with 14 storage nodes and 3 monitor nodes. The storage node have 12 SATA disks and 4 SSDs. 2 of the SSDs we plan to use as
journal disks and 2 for cache tiering.
Now the question raised in our team if it would be better to put all SSDs lets say in 2 storage nodes and consider them as fast nodes or to distribute the SSDs for the cache tiering over all 14 nodes (2 per node).
In mine opinion, if I understood the concept of Ceph right (I'm still in the learning process ;-) distributing the SSDs across all storage nodes would be better since this also would distribute the network traffic (client access) across all 14 nodes and not only limit it to 2 nodes. Right?
Any suggestion on that?
Best,
Martin
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