[solved] qemu slow file transfer from guest to host via smb

Joe S · 2018-01-28 12:34:05

Hello,

so I´ve been using qemu with kvm on my system and nearly everything works fine. But there is one anomaly that drives me crazy: If I copy a file from within the guest OS to a public writeable samba share on the host the transfer speed is just about 3 MiB/s!

- Using iperf I found the network speed from the guest OS to the host to be roughly 4 Gbit/s
- If I copy a file from the public samba share of the host to the guest OS (from within the guest OS) the transfer speed is about 80 MiB/s
- If I copy a file via samba from the host OS to a public writeable samba share on the same host the transfer speed is about 140 MiB/s (!)

So where is the bottleneck? Why is the transfer speed via samba from the guest OS to the host just 3 MiB/s?
Thanks in advance

Last edited by Joe S (2018-02-05 17:18:39)

Lone_Wolf · 2018-01-29 13:35:34

https://wiki.archlinux.org/index.php/QEMU#Networking
What type of network protocol is qemu using for your vm ?

Joe S · 2018-01-29 16:04:29

Lone_Wolf wrote:

https://wiki.archlinux.org/index.php/QEMU#Networking
What type of network protocol is qemu using for your vm ?

Hi, what do you mean by that? I tested the NIC in the VM with iperf (TCP) and I got nearly 4 Gbit/s. Shouldn't samba (which uses TCP) hence be quite fast?

Lone_Wolf · 2018-01-30 13:29:21

Keep in mind that the VM usually has a virtual network card, not a physical one.
Sending data to a virtual network card is akin to copying data from 1 loc in memory to another loc in that same memory.
When copying stuff through smb , other devices then memory (like hdd/ssd, usb drives etc) also influence the speed.

apart from that, qemu provides several networking methods for networking .
The default one is called user-mode networking .
It's simple but slow.

TAP networking is more complicated but a lot faster.

Then the type of OS running on the VM and the driver it uses also influences the speed.

Are you using user-mode networking, tap networking or some other method ?
If you're not certain, post the commandline you use to start the VM.
(if using virt-manager, check it's wiki page how to get that info).

Joe S · 2018-01-30 15:43:49

Lone_Wolf wrote:

Keep in mind that the VM usually has a virtual network card, not a physical one.
Sending data to a virtual network card is akin to copying data from 1 loc in memory to another loc in that same memory.
When copying stuff through smb , other devices then memory (like hdd/ssd, usb drives etc) also influence the speed.

Hi,

thanks for trying to help me, but I think that argument of yours is not true in this case:

I started the iperf server on my host and ran 5 tests from inside the VM (Arch Linux) to the iperf-server. The iperf-server prints the same results (~4 Gbit/s) on stdout as does the client inside the VM.

So, to debunk that myth that the iperf-server 'just copies data from 1 loc in memory to another' I copied data using ssh.
I created 1GiB of random data inside the VM (dd if=/dev/urandom of=testdata bs=1M count=1024) and copied that file with scp to the host with ~150 MiB/s (which is nearly 90% of the maximum bandwidth of my disk).

I tried optimizing the Samba server as suggested in the wiki but to no result: still just ~ 3MiB/s from guest OS to host ... (same disk I used in my scp test).

Greetz

Lone_Wolf · 2018-01-31 11:09:09

For clarity :

In my opinion the number iperf gave is not a good reflection of the transfer speed.
You do appear to have a problem with smb transfers though.

The scp copy test does give an idea what speed the network transfer can reach .

Are you using user-mode networking, tap networking or some other method ?

You haven't answered that question .

Also :
What are your Host and guest OS, which filesystems are used ?

Joe S · 2018-02-05 17:17:41

The culprit is the lightweight file manager pcmanfm (and nautilus) which uses gvfs-smb to mount cifs/samba shares and it seems that this gvfs-smb has some hardcoded tiny, little buffers which causes bad transfer speeds.

So when I mounted the samba/cifs share via console

 mount -t cifs -o guest //10.0.2.2/test /mnt/test

I could copy files from within the guest OS (arch linux) to the host (arch linux) with nearly 160 MiB/s (which is the limit of my hdd).

Lone_Wolf · 2018-02-25 15:50:49

Glad you solved it.
gvfs is used by many applications, have you tried reporting the issue to them ?

Arch Linux

#1 2018-01-28 12:34:05

[solved] qemu slow file transfer from guest to host via smb

#2 2018-01-29 13:35:34

Re: [solved] qemu slow file transfer from guest to host via smb

#3 2018-01-29 16:04:29

Re: [solved] qemu slow file transfer from guest to host via smb

#4 2018-01-30 13:29:21

Re: [solved] qemu slow file transfer from guest to host via smb

#5 2018-01-30 15:43:49

Re: [solved] qemu slow file transfer from guest to host via smb

#6 2018-01-31 11:09:09

Re: [solved] qemu slow file transfer from guest to host via smb

#7 2018-02-05 17:17:41

Re: [solved] qemu slow file transfer from guest to host via smb

#8 2018-02-25 15:50:49

Re: [solved] qemu slow file transfer from guest to host via smb

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