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I have a server running Proxmox and some Linux VMs running on it.

  • On the host, I get 200+ MB/s write to the disk
  • On a VM, I only get around 50-60 MB/s

Caching is disabled for all VMs. I'm using Virtio-SCSI as storage controller

Specs of the server:

SuperMicro 5018R-WR
Intel Xeon E5-2620v4
64GB memory
4x 1TB WD RE (WDC WD1003FBYZ-010FB0)
Software RAID 10 + dmcrypt + lvm(thin)

I know that mdraid is not supported by proxmox, but as I get the full speed on the host, I don't think that's the problem.

Any hints would be very appreciated.

Thanks, Patrick

patrick7
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    Are you using paravirtualized disks/controllers? – Spooler May 22 '18 at 16:38
  • What disk image format are you using? Can you show the output of `qemu-img info ` (with the guest powered off)? Can you try enabling writeback caching on the guest disk? – shodanshok May 22 '18 at 18:17
  • I'm using LVM-Thin for VMs. I have read that the recommendation is to use no cache, but I will try writeback and come back with a feedback. – patrick7 May 22 '18 at 18:19
  • ~190 MB/s with writeback cache. Why is the difference to none so big? And how about safety of writeback in case of a power outage? – patrick7 May 22 '18 at 20:22
  • This is why God invented the UPS. – Michael Hampton May 22 '18 at 20:32
  • That's why the server is colocated in a professional datacenter with 2 power feeds & UPS, but there's still a minimal risk :-) – patrick7 May 22 '18 at 20:36
  • Another question, why is writing without cache so slow? On a RAID 10 with 4 WD Gold I'd expect ~200MB/s without any caching. Also the FSYNC rate is very low (40-50/s). Probably related with the encryption layer? – patrick7 May 23 '18 at 09:08

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