Awful FreeBSD 9 block IO performance in KVM

[Richard Yao]

On 07/22/2012 03:19 AM, Wojciech Puchar wrote:
>> You are right. It is not capped at that speed:
>>
>> root at freebsd:/root # dd if=/dev/zero of=/dev/da1 bs=16384 count=262144
>>
>>
>>
>> 262144+0 records in
>> 262144+0 records out
>> 4294967296 bytes transferred in 615.840721 secs (6974153 bytes/sec)
>>
>>
> you did test da1 while dmesg are about da0?
> 
> is it OK and da1 is another qemu-kvm vdisk?
> 
> If so, check
> 
> dd if=/dev/zero of=/dev/da1 bs=512 count=256k
> 
> and compare speed.
> 
> i bet at something near 250kB/s and i think it is long I/O service 
> pathlength in qemu-kvm SCSI device simulator.
> 
> Just my bet i don't run FreeBSD on any VM (as opposed to running Windows 
> under FreeBSD in VBox)
> 
> check out how much CPU is used on the host side when you do that test.
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The test was on da1, but that does not explain how Linux manages to get
over 100MB/s when writing to a file on ext4.

I will do some more comprehensive tests as soon as I find time.

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