Disk Speeds inside a VM.

[rofo69]

This is my issue.

My proxmox host has 2 SSD's. One is for the host itself, and the other is for the virtual Disks for the VMs and containers.
If I run hdparm or dd directly on the host, I get speeds on the VM SSD disk of around 370-390 MB/s, which is what I would expect.

If I go into a VM (doesnt matter if its windows or Ubuntu), the write speeds drop to 60-70MB/s with caching off, and around 110-130MB/s with write back enabled.
And with write back enabled, I get file transfers pausing for a few seconds when copying large files (6-8 GB).

So why is the write speed so slow when comparing performance from the host itself, and from within a VM ?

 

[waltar]

Select for disk interface virtio block.

 

[rofo69]

I am already set as follows:-

 

[waltar]

Best do io on second disk in vm but could be defined on first also, virtio block is selected after virtio scsi single :

 

[rofo69]

Where do I find the setting for virtIO Block if the VM has already been created ?

 

[_gabriel]

zfs with consumer grade ?

 

[rofo69]

As for virtio block, it seems thats being deprecated according to this thread:-

https://forum.proxmox.com/threads/changing-scsi-controller-disk-controller.147495/

 

[rofo69]

Also, if it was the file system type slowing it down, why does it race along at normal speeds when testing from the host, but its slow in the guest?
I have other VM's running, but not doing huge amounts (usually mostly idle), but its possible there is contention on the drive, but the speed seems ridiculously slow.

Same thing happens inside an unpriv LXC too, speeds are slow.

 

[waltar]

As mentioned create second disk for your vm or lxc, choose virtio scsi single and virtio block, use it as block inside for eg a db or do a filesystem onto and bench that second disk for io.

 

[rofo69]

@waltar - I created a second disk on a windows VM and mounted it as drive D, using the settings above. The speeds were exactly the same (very slow).

 

[waltar]

Windows is anyway slow by design ...

 

[rofo69]

Same happens in ubuntu and a debian LXC.
I cant work out why the ssd disk is normal speed on the host but much much slower in the guest.

 

[waltar]

host raid6 5x2T, hdparm -t 300MB/s, Debian vm 2nd disk 620MB/s. Anyway hdparm is not a good benchmark program.