El 06/05/14 18:40, Christian Balzer escribi?: > Hello, > > On Tue, 06 May 2014 17:07:33 +0200 Xabier Elkano wrote: > >> Hi, >> >> I'm designing a new ceph pool with new hardware and I would like to >> receive some suggestion. >> I want to use a replica count of 3 in the pool and the idea is to buy 3 >> new servers with a 10-drive 2,5" chassis each and 2 10Gbps nics. I have >> in mind two configurations: >> > As Wido said, more nodes are usually better, unless you're quite aware of > what you're doing and why. Yes, I know that, but what is the minimum number of nodes to start with? Start with three nodes is not a feasible option? > >> 1- With journal in SSDs >> >> OS: 2xSSD intel SC3500 100G Raid 1 >> Journal: 2xSSD intel SC3700 100G, 3 journal for each SSD > As I wrote just a moment ago, use at least the 200GB ones if performance > is such an issue for you. > If you can afford it, use 4 3700s and share OS and journal, the OS IOPS > will not be that significant, especially if you're using a writeback cache > controller. the journal can be shared with the OS, but I like the RAID 1 for the OS. I think that the only drawback with it is that I am using two dedicated disk slots for OS. > >> OSD: 6 SAS10K 900G (SAS2 6Gbps), each running an OSD process. Total size >> for OSDs: 5,4TB >> >> 2- With journal in a partition in the spinners. >> >> OS: 2xSSD intel SC3500 100G Raid 1 >> OSD+journal: 8 SAS15K 600G (SAS3 12Gbps), each runing an OSD process and >> its journal. Total size for OSDs: 3,6TB >> > I have no idea why anybody would spend money on 12Gb/s HDDs when even > most SSDs have trouble saturating a 6Gb/s link. > Given the double write penalty in IOPS, I think you're going to find > this more expensive (per byte) and slower than a well rounded option 1. But these disks are 2,5" 15K, not only for the link. Other SAS 2,5" (SAS2) disks I found are only 10K. The 15K disks should be better for random IOPS. > >> The budget in both configuration is similar, but the total capacity not. >> What would be the best configuration from the point of view of >> performance? In the second configuration I know the controller write >> back cache could be very critical, the servers has a LSI 3108 controller >> with 2GB Cache. I have to plan this storage as a KVM image backend and >> the goal is the performance over the capacity. >> > Writeback cache can be very helpful, however it is not a miracle cure. > Not knowing your actual load and I/O patterns it might very well be > enough, though. The IO patterns are a bit unknown, I should assume 40% read and 60% write, but the IO size is unknown, because the storage is for KVM images and the VMs are for many customers and different purposes. > > Regards, > > Christian
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