Windows KVM slow write speed (not over 40 MB/s)

[silbro]

Hey all

I tried so many settings with my Windows KVM and I can never get over 40MB/s. I don't know what I'm doing wrong. I'm using the newest virtio drivers, I tried the local disk and an NFS share on freenas. I have used SSDs on the freenas in a raid1 and I still can't get over 40MB/s. I tried different mount options (rw,tcp,rsize=[x],wsize=[x],noatime etc), different harddrive settings (qcow, raw, writeback etc.). Has anyone gotten a NFS share and actually has the network being the bottleneck? In my opinion I'm supposed to get up to 100 - 120 MB/s over my 1 gig network. My linux systems actually write with up to 120MB/s without a problem.

Any tips would really be appreciated as I have tried so many options. If you need info, I will of course give you that.

Thanks in advance for any help!
silbro

 

[silbro]

Nobody here ever had this problem ?

 

[nethfel]

I don't have the problem you're referring to on a local drive - last I tried a speed test on disk write in a kvm for a local disk it was very, very fast (using a performance benchmark tool from Hiren's while in Windows 7).

I wouldn't expect you to get near the theoretical max of a gb ethernet (~125MB/s) through nfs - most I've seen on benchmarks are between 70 and 100 MB/s between pure host and client. I'd imagine that there would be some overhead on a virtualized environment that would reduce the throughput a bit, especially if you have more than one VM image on that NFS host that is running...

 

[udo]

Nobody here ever had this problem ?

Hi,
I don't use nfs-shares for VMs.

But two points, where you can looking for:

1. Is your MTU correct? On both sides 1500?

2. Do you use an bridged network for the nfs-storage? Perhaps it's better to use an dedicate (unbridged nic) for the nfs-traffic.
In this case it's also easier to use jumbo frames (which don't work with bridges, except openvswitch).

Udo

 

[Aristotlejones]

What is your network situation look like? Are the vmdk's stored on an nfs share? And more importantly, how much ram is in your freenas box? I find that if I want to get the max i/o from freenas I need lots of ram. Currently 32gigs is the absolute minimum I'd use when striping across mirrors. Raidz needs even more imho. How many nice are in your host and freenas box? What sort of switch do you have? Jumbo frame support?

Cheers


Sent from my iPad using Tapatalk

 

[silbro]

Hi nethfel

last I tried a speed test on disk write in a kvm for a local disk it was very, very fast (using a performance benchmark tool from Hiren's while in Windows 7).

Ok, maybe I should have pointed out that it's only on SATA drives either shared over a network or locally. With a RAID controller I also had extremely fast write speeds, but that was on SSD drives and also local.

But at least on SATA drives you should get around 50-90MB/s. I find it interesting that with linux I get almost 122MB/s and Windows is so much worse. I've used virtio drivers in both cases. Overhead on Windows can't be that much worse than linux... or can it be?

 

[silbro]

1. Is your MTU correct? On both sides 1500?

I need to check that first. But I thought it was.

2. Do you use an bridged network for the nfs-storage? Perhaps it's better to use an dedicate (unbridged nic) for the nfs-traffic.
In this case it's also easier to use jumbo frames (which don't work with bridges, except openvswitch).

Hmm that is a good point, I actually am using the shared one instead of a direct connection. I got 2 network cards per server, so I put them in failover. (NAS is on robin round though with 2 cards)

 

[silbro]

Hey Aristotlejones

Good questions. I only have 16GB of RAM on my freenas. I am actually striping across mirrors.

What do you mean by nice? If you meant NIC i have 2 on each node and 4total on my freenas (using only 2 atm).

My switch is a Netgear ProSafe GS116E v2. I don't think jumbo frames are supported, maybe I'm wrong. (Manual)

Everything is connected to 2 of those switches. I'm also using a USG 100 from Zyxel.

cheers

 

[nethfel]

That's why I'm wondering if there is something else going on. I was seeing between 80 and 110 on sata drives on my server (when the vm was the only one on that drive and I was not running any other vms that might interfere with it). You should get higher then 40 MB/s on a local drive based VM, especially if it's the only VM running...

 

[Aristotlejones]

Just to be clear, you are writing to the Windows Virtual machine from another machine?

 

[nethfel]

Just to be clear, you are writing to the Windows Virtual machine from another machine?

This is ability of throughput within the vm to its vhd. In the case I was referring to it's throughput of a running machine on a single box with the vhd actually stored on a separate drive within the box. The OP should be seeing more than 40MB/s in that sort of configuration (which he mentioned he wasn't seeing that throughput with the vm stored locally and that his local performance was about the same as his performance with the vhd being stored on an NFS share - unless I completely misunderstood the OP)

 

[silbro]

Just to be clear, you are writing to the Windows Virtual machine from another machine?

I'm writing from within the VM itself to it's own virtual harddrive. Because I'm using free NAS as a shared storage and have 3 nodes I'm doing everything over the network (if you look at it from that perspective). But essentially I just copy a big file locally onto the vm itself. I guess that is my speed test

 

[nethfel]

Ok, did you also test with the VHD not on the NAS? From your original post, I read it as you had tested it with the VM stored on a local HDD and not just the NAS.

 

[silbro]

Ok, did you also test with the VHD not on the NAS? From your original post, I read it as you had tested it with the VM stored on a local HDD and not just the NAS.

Yes i also tested it with a local SATA2 drive. I reach around 30-43 MB/s copying a big file.