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  • 1.  NVMe Cache Tier (All Flash Capacity Tier) - Write Buffer issues?

    Posted Jun 22, 2017 12:35 PM

    Hi there,

    I am very new to the vSAN topic. Started toying around with it a week ago on my home lab. Run into several issues. But now it seams up and running. At least thats what vSAN health tells me :smileyhappy:

    Anyway as described in the topic I am running a 2 Node ROBO Setup with two similar supermicro servers both 1x Samsung 960 Pro (1TB NVMe) and 5x Samsung 850 Pro (512GB SSDs). For some testing I deployed a W2k16 Datacenter Edition on the vSAN Datastore and ran some benchmark (see the attached picture). Which brings me to the question is it possible that Read Cache is working... but Write Cache is not?

    Maybe its misconfigurated, maybe I did not understand the concept of vSAN correctly. But I really like to know what I am doing wrong. Or where my thinking goes wrong :smileyhappy:

    Thanks a lot for your help in advance.



  • 2.  RE: NVMe Cache Tier (All Flash Capacity Tier) - Write Buffer issues?

    Posted Jul 11, 2017 10:56 PM

    I think the problem is the 960Pro isn't an enterprise-class SSD.  This article is about the problem with consumer-grade SSD devices in Windows-land, but I think it applies here:

    Don’t do it: consumer-grade solid-state drives (SSD) in Storage Spaces Direct | Storage at Microsoft

    Baiscally VSAN tries to guarantee these cached writes are happening, so it sends them to the cache-tier SSD with a write-through command, meaning the SSD has to actually write them instead of just (again) caching them itself.