[ovirt-users] Poor guest write speeds

[Alan Murrell]

On 27/07/2015 6:54 PM, Donny Davis wrote:
> Add this on to your dd command
>
> conv=sync

On the console login of my host/server, the 'dd' command I used was:

dd bs=1M count=256 if=/dev/zero of=test conv=fdatasync

Would I be replacing the 'conv=fdatasync' with the 'conv=sync', or am I 
appending it (i.e., 'conv=fdatasync,sync')

> And then report result. Its no surprise that your writes are slow in raid1

I will give it a try and report results when I re-install my 
oVirt+Hosted Engine on my host, however it actually, wasn't/isn't the 
write speed on the bare metal host that I was too concerned with but 
rather the poor write speed within the guest VMs, particularly my 
Windows 7 guest.

As you can see from my original results, within the Win7 guest on oVirt 
I was only getting about 10 MByte/s while the host I was able to get 
about 74 MByte/s.  That just seems like one heck of a performance 
drop-off to me, which means I either don't have something 
configured/tuned properly, or virtio-SCSI is not that good (though 
everything I have rad suggests that virtio-SCSI is what I should be 
using for best performance, as long as the guest drivers are installed)

-Alan


>
> Hello,
>
> I am running oVirt 3.5 on a single server (hosted engine).  I have two
> Western Digital WD20EZRX drives in a hardware RAID1 configuration.  My
> storage is actually on the single server, but I am attaching to it via NFS.
>
> I created a Windows 7 guest, and I am finding its write speeds to be
> horrible.  It is a VirtIO-SCSI drive and the guest additions are installed.
>
> The installation of the OS took way longer than bare metal or even
> VMware.  When I ran Windows updates, it again took a *lot* longer than
> on bar metal or on VMware.
>
> The read speeds seem to be fine.  The guest is responsive when I click
> on programs and they open about as fast as bare metal or VMware.
>
> I downloaded and ran "Parkdale" HDD tester and ran a test with the
> following settings:
>
>    - File size: 4000
>    - Block Size: 1 MByte
>
> The results are as follows:
>
>    - Seq. Write Speed: 10.7 MByte/sec  (Random Q32D: )
>    - Seq. Read Speed: 237.3 MByte/sec  (Random Q32D: )
>
> I ran another test, but this time changing the "Block Size" to "64 kByte
> [Windows Default]".  Results are as follows:
>
>    - Seq. Write Speed: 10.7 MByte/sec  (Random Q32D: )
>    - Seq. Read Speed: 237.3 MByte/sec  (Random Q32D: )
>
> On the host, running '|dd bs=1M count=256 if=/dev/zero of=test
> conv=fdatasync|' on my data mount via NFS rsuled in the following:
>
> 256+0 records in
> 256+0 records out
> 268435456 bytes (268 MB) copied, 3.59431 s, 74.7 MB/s
>
> I got this <https://romanrm.net/dd-benchmark>
> <https://romanrm.net/dd-benchmark> and measures the write speed of a
> disk.  As you can see, it is significantly higher than what I am getting
> in the Windows guest VM.
>
> Running that same "dd" test on an Ubuntu guest VM gives me 24MB/s.
>
> Any ideas why I have such poor write performance?  Is this normal with
> oVirt guests?  Any ideas on what I might be able to do to improve them?
> I don't expect to get close to the "bare metal" results, but maybe
> something in the 40-60 MB/s range would be nice.
>
> Thanks, in advance, for your help and advice.
>
> -Alan
>
>
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