Re: bad O_DIRECT read and write performance with small block sizes with virtio

[Avi Kivity <avi@redhat.com>]

  On 08/03/2010 05:57 PM, John Leach wrote:
> On Tue, 2010-08-03 at 17:44 +0300, Avi Kivity wrote:
>> On 08/03/2010 05:40 PM, John Leach wrote:
>>> dd if=/dev/mapper/zero of=/dev/null bs=8k count=1000000 iflag=direct
>>> 8192000000 bytes (8.2 GB) copied, 3.46529 s, 2.4 GB/s
>>>
>>> dd if=/dev/mapper/zero of=/dev/null bs=8k count=1000000
>>> 8192000000 bytes (8.2 GB) copied, 5.5741 s, 1.5 GB/s
>>>
>>> dd is just using read.
>>>
>> What's /dev/mapper/zero?  A real volume or a zero target?
>>
> zero target:
>
> echo "0 21474836480 zero" | dmsetup create zero
>
> The same performance penalty occurs when using real disks though, I just
> moved to a zero target to rule out the variables of spinning metal and
> raid controller caches.

Don't, it's confusing things.  I'd expect dd to be slower with 
iflag=direct since the kernel can't do readahead an instead must 
roundtrip to the controller.  With a zero target it's faster since it 
doesn't have to roundtrip and instead avoids a copy.

-- 
error compiling committee.c: too many arguments to function